Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities by Contractual Maturity (Total Held-to-maturity Securities) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investment [Line Items]
Total, Amortized Cost Basis	$ 550,153	$ 583,287
Total, Fair Value	576,989	627,349
Carrying Amount [Member]
Investment [Line Items]
Less than one year, Amortized Cost Basis	63,140
One to five years, Amortized Cost Basis	145,838
Five to ten years, Amortized Cost Basis	130,159
More than ten years, Amortized Cost Basis	132,205
Total, Amortized Cost Basis	550,153	583,287
Fair Value [Member]
Investment [Line Items]
Less than one year, Fair Value	63,427
One to five years, Fair Value	153,601
Five to ten years, Fair Value	140,059
More than ten years, Fair Value	135,219
Total, Fair Value	576,989
U.S. agency-based mortgage-backed securities [Member]
Investment [Line Items]
Total, Amortized Cost Basis	23,876	32,984
Total, Fair Value	25,842	36,042
U.S. agency-based mortgage-backed securities [Member] | Carrying Amount [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	23,876
U.S. agency-based mortgage-backed securities [Member] | Fair Value [Member]
Investment [Line Items]
Held-to-maturity, Fair Value	25,842
Commercial mortgage-backed securities [Member]
Investment [Line Items]
Total, Amortized Cost Basis	51,414	51,529
Total, Fair Value	55,275	57,962
Commercial mortgage-backed securities [Member] | Carrying Amount [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	51,414
Commercial mortgage-backed securities [Member] | Fair Value [Member]
Investment [Line Items]
Held-to-maturity, Fair Value	55,275
Asset-backed securities [Member]
Investment [Line Items]
Total, Amortized Cost Basis	3,521	4,229
Total, Fair Value	3,566	4,245
Asset-backed securities [Member] | Carrying Amount [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	3,521
Asset-backed securities [Member] | Fair Value [Member]
Investment [Line Items]
Held-to-maturity, Fair Value	$ 3,566