Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Investment [Line Items]
Due in 2015, Amortized Cost Basis	$ 59,564
In 2016 through 2019, Amortized Cost Basis	214,075
In 2020 through 2024, Amortized Cost Basis	52,827
After 2024, Amortized Cost Basis	108,850
Total , Amortized Cost Basis	447,988	$ 376,109
Due in 2015, Fair Value	59,891
In 2016 through 2019, Fair Value	217,789
In 2020 through 2024, Fair Value	54,534
After 2024, Fair Value	116,468
Total, Fair Value	460,697	$ 380,022
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-sale securities, Amortized Cost Basis	12,672
Available-for-sale securities, Fair value	$ 12,015