Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Investment [Line Items]
Less than one year, Amortized Cost	$ 55,153
One to five years, Amortized Cost	186,929
Five to ten years, Amortized Cost	140,954
More than ten years, Amortized Cost	124,529
Total, Amortized Cost	579,145	536,583
Less than one year, Fair Value	55,726
One to five years, Fair Value	194,771
Five to ten years, Fair Value	150,762
More than ten years, Fair Value	129,891
Total, Fair Value	607,512	561,179
U.S. agency-based mortgage-backed securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	19,170
Held-to-maturity, Fair Value	21,062
Commercial mortgage-backed securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	49,327
Held-to-maturity, Fair Value	52,094
Asset-backed securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	3,083
Held-to-maturity, Fair Value	$ 3,206