Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Within one year, Amortized Cost Basis	$ 109,707
After one year through five years, Amortized Cost Basis	259,942
After five years through ten years, Amortized Cost Basis	105,396
After ten years, Amortized Cost Basis	75,425
Held-to-Maturity, Amortized Cost	562,434	$ 645,164
Within one year, Fair Value	110,469
After one year through five years, Fair Value	263,620
After five years through ten years, Fair Value	106,341
After ten years, Fair Value	75,485
Total, Fair Value	568,931	$ 662,276
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost Basis	9,967
Held-to-maturity, Fair Value	10,915
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost Basis	70
Held-to-maturity, Fair Value	70
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost Basis	1,927
Held-to-maturity, Fair Value	$ 2,031