Investments - Schedule of Fixed Maturities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fixed Maturities, Available for Sale, Amortized Cost
Due in one year or less	$ 132,153
Due from one to five years	510,813
Due from five to ten years	931,954
Due from ten to twenty years	3,739,429
Due after twenty years	7,560,176
Mortgage-backed and asset-backed securities	88,305
Fixed Maturities, Available for Sale, Amortized Cost	12,962,830	12,823,612
Fixed Maturities, Available for Sale, Fair Value
Due in one year or less	134,358
Due from one to five years	575,083
Due from five to ten years	1,043,718
Due from ten to twenty years	4,379,923
Due after twenty years	8,662,450
Mortgage-backed and asset-backed securities	98,982
Fixed Maturities, Available for Sale, Fair Value	$ 14,894,514	$ 14,493,060