Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Within one year, Amortized Cost	$ 91,818
After one year through five years, Amortized Cost	252,775
After five years through ten years, Amortized Cost	90,966
After ten years, Amortized Cost	160,120
Held-to-maturity, Amortized Cost, Totals	605,877	$ 629,668
Within one year, Fair Value	92,103
After one year through five years, Fair Value	252,787
After five years through ten years, Fair Value	91,281
After ten years, Fair Value	160,339
Held-to-maturity, Fair Value, Totals	607,072	639,309
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	9,038
Held-to-maturity, Amortized Cost, Totals	9,038	10,260
Held-to-maturity, Fair Value	9,387
Held-to-maturity, Fair Value, Totals	9,387	10,845
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	1,160
Held-to-maturity, Amortized Cost, Totals	1,160	1,299
Held-to-maturity, Fair Value	1,175
Held-to-maturity, Fair Value, Totals	$ 1,175	$ 1,309