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, 2016	Dec. 31, 2015
Investment [Line Items]
Within one year, Amortized Cost Basis	$ 64,852
After one year through five years, Amortized Cost Basis	215,486
After five years through ten years, Amortized Cost Basis	30,846
After ten years, Amortized Cost Basis	158,259
Total , Amortized Cost Basis	479,871	$ 376,109
Within one year, Fair Value	65,174
After one year through five years, Fair Value	215,624
After five years through ten years, Fair Value	30,488
After ten years, Fair Value	158,469
Total, Fair Value	479,097	$ 380,022
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-sale securities, Amortized Cost Basis	10,428
Available-for-sale securities, Fair value	$ 9,342