Private U.S. property and casualty insurers' net income after taxes grew to $44 billion in the first nine months of 2015, from $37.8 billion in the same period in 2014, with insurers' overall profitability as measured by their rate of return on average policyholders' surplus growing to 8.8 percent from 7.6 percent, according to a report from Jersey City, N.J.-based ISO and Chicago-based Property Casualty Insurers Association of America (PCI).

Insurers' combined ratio improved to 96.9 percent for the first nine months of 2015 from 97.7 percent in the same period of 2014. Net written premium growth increased to 4.1 percent for the first nine months of 2015 from 4 percent for the same period in 2014. Net investment income increased to $34.8 billion for the period from $34.5 billion a year earlier, and realized capital gains increased slightly to $8.9 billion from $8.8 billion, resulting in $43.7 billion in net investment gains for the first nine months of 2015.

"Insurers overall had another strong quarter. Surplus and premium to surplus continue to hover near historic levels, underscoring insurers' rock-solid financial foundation and ability to serve consumers," said Robert Gordon, PCI's senior vice president for policy development and research. "However, premium growth continues to be sluggish for commercial lines. Additionally, some industry statistics we're monitoring indicate that the combined ratio for personal auto insurance has worsened, driven by increases in both accident severity and frequency."