Investments - Summary of the Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Investment [Line Items]
Less than one year, Amortized Cost	$ 55,114
One to five years, Amortized Cost	171,828
Five to ten years, Amortized Cost	127,493
More than ten years, Amortized Cost	120,626
Total, Amortized Cost	548,612	536,583
Less than one year, Fair Value	55,575
One to five years, Fair Value	179,609
Five to ten years, Fair Value	136,840
More than ten years, Fair Value	124,646
Total, Fair Value	575,175	561,179
U.S. agency-based mortgage-backed securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	20,418
Held-to-maturity, Fair Value	22,212
Commercial mortgage-backed securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	49,884
Held-to-maturity, Fair Value	52,963
Asset-backed securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	3,249
Held-to-maturity, Fair Value	$ 3,330