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, 2018	Dec. 31, 2017
Investment [Line Items]
Within one year, Amortized Cost	$ 97,545
After one year through five years, Amortized Cost	255,343
After five years through ten years, Amortized Cost	94,872
After ten years, Amortized Cost	161,018
Held-to-maturity, Amortized Cost, Totals	619,400	$ 629,668
Within one year, Fair Value	97,835
After one year through five years, Fair Value	255,678
After five years through ten years, Fair Value	95,139
After ten years, Fair Value	161,212
Held-to-maturity, Fair Value, Totals	620,936	$ 639,309
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	9,400
Held-to-maturity, Fair Value	9,831
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	1,222
Held-to-maturity, Fair Value	$ 1,241