Schedule of Fixed Maturities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investments Debt And Equity Securities [Abstract]
Fixed maturities available for sale, Amortized Cost - Due in one year or less	$ 106,388
Fixed maturities available for sale, Amortized Cost - Due from one to five years	502,441
Fixed maturities available for sale, Amortized Cost - Due from five to ten years	881,514
Fixed maturities available for sale, Amortized Cost - Due from ten to twenty years	2,849,127
Fixed maturities available for sale, Amortized Cost - Due after twenty years	7,763,876
Mortgage-backed and asset-backed securities, Amortized Cost	107,593
Total fixed maturities available for sale, Amortized Cost	12,210,939	11,963,406	[1]
Fixed maturities available for sale, Fair Value - Due in one year or less	108,421
Fixed maturities available for sale, Fair Value - Due from one to five years	546,550
Fixed maturities available for sale, Fair Value - Due from five to ten years	953,025
Fixed maturities available for sale, Fair Value - Due from ten to twenty years	3,123,815
Fixed maturities available for sale, Fair Value - Due after twenty years	8,034,791
Mortgage-backed and asset-backed securities, Fair Value	95,965
Total fixed maturities available for sale, Fair Value	$ 12,862,567	$ 13,541,193	[1]

[1] Derived from audited financial statements