Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Within one year, Amortized Cost	$ 71,596
After one year through five years, Amortized Cost	164,449
After five years through ten years, Amortized Cost	38,945
After ten years, Amortized Cost	168,783
Total, Amortized Cost	462,033	$ 461,236
Within one year, Fair Value	71,476
After one year through five years, Fair Value	162,658
After five years through ten years, Fair Value	38,371
After ten years, Fair Value	169,774
Total, Fair Value	459,264	$ 465,594
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-sale securities, Amortized Cost	18,260
Available-for-sale securities, Fair value	$ 16,985