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, 2016	Dec. 31, 2015
Investment [Line Items]
Within one year, Amortized Cost Basis	$ 98,055
After one year through five years, Amortized Cost Basis	305,347
After five years through ten years, Amortized Cost Basis	113,508
After ten years, Amortized Cost Basis	85,345
Held-to-Maturity, Amortized Cost	639,632	$ 645,164
Within one year, Fair Value	98,946
After one year through five years, Fair Value	313,740
After five years through ten years, Fair Value	118,689
After ten years, Fair Value	88,958
Total, Fair Value	659,172	$ 662,276
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost Basis	12,332
Held-to-maturity, Fair Value	13,597
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost Basis	22,855
Held-to-maturity, Fair Value	22,948
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost Basis	2,190
Held-to-maturity, Fair Value	$ 2,294