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, 2017	Dec. 31, 2016
Investment [Line Items]
Within one year, Amortized Cost	$ 120,697
After one year through five years, Amortized Cost	235,710
After five years through ten years, Amortized Cost	112,733
After ten years, Amortized Cost	119,037
Held-to-maturity, Amortized Cost, Totals	599,096	$ 562,434
Within one year, Fair Value	121,160
After one year through five years, Fair Value	240,121
After five years through ten years, Fair Value	115,665
After ten years, Fair Value	121,768
Held-to-maturity, Fair Value, Totals	610,551	$ 568,931
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	9,171
Held-to-maturity, Fair Value	9,966
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	1,748
Held-to-maturity, Fair Value	$ 1,871