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, 2015	Dec. 31, 2014
Investment [Line Items]
Within one year, Amortized Cost Basis	$ 146,632
After one year through five years, Amortized Cost Basis	280,016
After five years through ten years, Amortized Cost Basis	137,031
After ten years, Amortized Cost Basis	72,721
Held-to-Maturity, Amortized Cost	698,513	$ 639,631
Within one year, Fair Value	147,370
After one year through five years, Fair Value	287,865
After five years through ten years, Fair Value	142,228
After ten years, Fair Value	74,298
Total, Fair Value	716,856	$ 664,371
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost Basis	14,974
Held-to-maturity, Fair Value	16,488
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost Basis	44,907
Held-to-maturity, Fair Value	45,908
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost Basis	2,532
Held-to-maturity, Fair Value	$ 2,699