Investments - Schedule of Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fixed Maturities, Available for Sale, Amortized Cost
Due in one year or less	$ 72,803
Due from one to five years	533,791
Due from five to ten years	972,641
Due from ten to twenty years	3,827,375
Due after twenty years	7,633,945
Mortgage-backed and asset-backed securities	86,363
Fixed Maturities, Available for Sale, Amortized Cost	13,126,918	$ 12,823,612
Fixed Maturities, Available for Sale, Fair Value
Due in one year or less	73,790
Due from one to five years	592,958
Due from five to ten years	1,063,069
Due from ten to twenty years	4,267,617
Due after twenty years	8,027,399
Mortgage-backed and asset-backed securities	96,629
Fixed Maturities, Available for Sale, Fair Value	$ 14,121,462	$ 14,493,060