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, 2017	Dec. 31, 2016
Investment [Line Items]
Within one year, Amortized Cost Basis	$ 77,440
After one year through five years, Amortized Cost Basis	181,269
After five years through ten years, Amortized Cost Basis	36,875
After ten years, Amortized Cost Basis	162,323
Total , Amortized Cost Basis	467,989	$ 479,871
Within one year, Fair Value	77,792
After one year through five years, Fair Value	181,818
After five years through ten years, Fair Value	36,735
After ten years, Fair Value	163,162
Total, Fair Value	468,690	$ 479,097
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-sale securities, Amortized Cost Basis	10,082
Available-for-sale securities, Fair value	$ 9,183