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, 2017	Dec. 31, 2016
Investment [Line Items]
Within one year, Amortized Cost	$ 92,665
After one year through five years, Amortized Cost	164,477
After five years through ten years, Amortized Cost	35,020
After ten years, Amortized Cost	155,059
Total , Amortized Cost	459,116	$ 479,871
Within one year, Fair Value	92,899
After one year through five years, Fair Value	165,259
After five years through ten years, Fair Value	35,254
After ten years, Fair Value	158,391
Total, Fair Value	462,793	$ 479,097
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-sale securities, Amortized Cost	11,895
Available-for-sale securities, Fair value	$ 10,990