Investments - Summary of Amortized Cost and Estimated Fair Value of Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
One year or less, Amortized Cost	$ 20,271
More than one year through two years, Amortized Cost	13,348
More than two years through five years, Amortized Cost	69,041
More than five years through ten years, Amortized Cost	115,637
More than ten years, Amortized Cost	569,893
Amortized cost due in years, Total	788,190
One year or less, Fair Value	20,281
More than one year through two years, Fair Value	13,376
More than two years through five years, Fair Value	70,127
More than five years through ten years, Fair Value	115,896
More than ten years, Fair Value	577,962
Fair value due in years, Total	$ 797,642	$ 769,071
One year or less, Percentage of Total Fair Value	2.50%
More than one year through two years, Percentage of Total Fair Value	1.70%
More than two years through five years, Percentage of Total Fair Value	8.80%
More than five years through ten years, Percentage of Total Fair Value	14.50%
More than ten years, Percentage of Total Fair Value	72.50%
Percentage of Fair Value, Total	100.00%