Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Within one year, Amortized Cost	$ 113,488
After one year through five years, Amortized Cost	239,333
After five years through ten years, Amortized Cost	98,935
After ten years, Amortized Cost	134,328
Held-to-maturity, Amortized Cost, Totals	598,658	$ 562,434
Within one year, Fair Value	113,988
After one year through five years, Fair Value	243,851
After five years through ten years, Fair Value	101,450
After ten years, Fair Value	137,424
Held-to-maturity, Fair Value, Totals	610,151	$ 568,931
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	10,928
Held-to-maturity, Fair Value	11,676
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	1,646
Held-to-maturity, Fair Value	$ 1,762