Yet, as insurers performed better in the past year in terms of results, there was little movement in the rankings of the top 10 companies or groups by net premiums written.

The top 100 groups showed average growth in NPW of 4.3%, to a total of $422.4 billion; that average growth drops to 3.7% among the top 5 groups (State Farm, Berkshire Hathaway, Allstate, Liberty Mutual and Travelers, respectively), which still represent a sizable portion (roughly 37%) of the NPW among the top 100 insurance groups.

State Farm's $55.4 billion in NPW, which is up 4.6% compared to 2012, still far outpaces the competition. Second-place Berkshire Hathaway reports $28.4 billion, a 4.1% gain, while Allstate in third grew NPW by 3.6% to $26.5 billion.

The top 10 groups were largely unchanged from 2012; USAA Insurance Group and Farmers Insurance Group of Companies switched spots at the bottom of the top 10, with USAA taking over No. 9 and Farmers dropping to No. 10. After the previously mentioned top 5 groups, AIG, Nationwide Mutual Group and Progressive Corp. filled in the 6-8 spots.

San Diego-based ICW Group ranks the highest in our list of net premium growth Leaders, with 40.1% growth year over year among the top 100 insurance groups and has moved up the list from No. 100 to No. 97. Its gains are attributable to aggressive efforts in workers' comp risk mitigation and leveraging its technology investment via its underwriting management system and agent portal, Snap, which has allowed it greater speed and efficiency in viewing, quoting and binding policies.

Private mortgage insurance company Radian Group saw some traction in the top 100 groups list, moving up from No. 78 to No. 68 (a jump in the top 100 groups list mirrored only by California's State Compensation Insurance Fund, which also moved up 10 spots to No. 60). Radian wrote 27% more new mortgage insurance business in 2013 versus 2012 ($47 billion vs $37 billion), expanding its customer base—particularly among small/midsized lenders, community banks and credit unions—and recapturing market share from the Federal Housing Administration, which offers government-backed mortgage insurance.

Another big gainer: Starr International Co., which exhibited 36% growth in NPW and moved up the list from No. 87 to No. 72 thanks to its broad and diverse product portfolio, continued recruitment of industry talent, and office expansion, both in the U.S. and internationally.

For company rankings, State Farm Mutual Automobile Ins. continues to lead the pack at $35.2 billion in NPW, followed by Allstate Insurance Co. at $25.1 billion, State Farm Fire & Casualty Co. at $15.3 billion and Nationwide Mutual Insurance Co. $14.6 billion. That top four is unchanged from 2012. Liberty Mutual Insurance Co. placed No. 5, switching places with Government Employees Ins. Co., which dropped to No. 6.

Farmers Insurance Exchange moved from No. 8 in 2012 to No. 7 in 2013. Chubb's Federal Insurance Co. also gained a spot to rank No. 8, as did CNA Financial Corp.'s Continental Casualty company, which improved to No. 9.

USAA's United Services Automobile broke into the top 10, moving up a spot from No. 11 to No. 10. Berkshire Hathaway's National Indemnity was the only company to fall out of the top 10, dropping seven spots to place No. 14 in 2013.

The top movers on the company list include two Liberty Mutual subsidiaries, Peerless Ins. Co. (which moved up the list from No. 33 to No. 12), and Employers Ins. Co. of Wausau (Wisc.), which moved up to No. 38 from No. 75. This can be attributed to Liberty Mutual's termination of the longstanding Peerless Insurance Pool, after which the pool's participants were added to the Liberty Pool, led by Liberty Mutual Insurance Co. Under the terms of the pooling agreement, effective Jan. 1. 2013, Peerless Ins. Co. and Employers Ins. Co. of Wausau (Wisc.) receive pooling percentages of 20% and 8%, respectively, driving the companies' changes in rank year over year.

Berkley Insurance Co. moved up a no-less-impressive 32 slots to No. 13 from No. 45, due in part to a large intercompany pooling arrangement introduced back on Jan. 1, 2013 in which previous insurer groups including Berkley Insurance Group, Berkley Regional Group, Admiral Group and Nautilus Group were collapsed and essentially all of Berkley's U.S. subsidiaries were combined into one large pool.

NEXT: Check out NU's Top 100 charts in full, starting here.

Note: The Top 100 groups are ranked based on net premiums written only for U.S.-based parent companies and their U.S.-domiciled subsidiaries. Premiums written by subsidiaries based outside the U.S. are not included in the net premiums written totals.