Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Within one year, Amortized Cost	$ 85,898
After one year through five years, Amortized Cost	160,278
After five years through ten years, Amortized Cost	32,117
After ten years, Amortized Cost	171,191
Total , Amortized Cost	462,892	$ 479,871
Within one year, Fair Value	86,051
After one year through five years, Fair Value	161,077
After five years through ten years, Fair Value	32,427
After ten years, Fair Value	175,033
Total, Fair Value	467,256	$ 479,097
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-sale securities, Amortized Cost	13,408
Available-for-sale securities, Fair value	$ 12,668