Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Within one year, Amortized Cost Basis	$ 114,829
After one year through five years, Amortized Cost Basis	252,703
After five years through ten years, Amortized Cost Basis	99,123
After ten years, Amortized Cost Basis	117,121
Held-to-Maturity, Amortized Cost	594,776	$ 562,434
Within one year, Fair Value	115,334
After one year through five years, Fair Value	257,432
After five years through ten years, Fair Value	100,770
After ten years, Fair Value	117,789
Total, Fair Value	603,295	$ 568,931
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost Basis	9,163
Held-to-maturity, Fair Value	10,024
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost Basis	1,837
Held-to-maturity, Fair Value	$ 1,946