Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Within one year, Amortized Cost	$ 76,194
After one year through five years, Amortized Cost	153,483
After five years through ten years, Amortized Cost	43,442
After ten years, Amortized Cost	169,304
Total, Amortized Cost	461,236	$ 479,871
Within one year, Fair Value	76,211
After one year through five years, Fair Value	153,092
After five years through ten years, Fair Value	43,606
After ten years, Fair Value	174,671
Total, Fair Value	465,594	$ 479,097
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-sale securities, Amortized Cost	18,813
Available-for-sale securities, Fair value	$ 18,014