Investments - Schedule of Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fixed Maturities Available for Sale, Amortized Cost
Due in one year or less	$ 53,589
Due from one to five years	572,013
Due from five to ten years	951,645
Due from ten to twenty years	3,841,490
Due after twenty years	7,662,273
Mortgage-backed and asset-backed securities	85,238
Fixed Maturities, Available for Sale, Amortized Cost	13,166,248	$ 12,823,612
Fixed Maturities Available for Sale, Fair Value
Due in one year or less	54,217
Due from one to five years	632,991
Due from five to ten years	1,042,986
Due from ten to twenty years	4,296,570
Due after twenty years	7,958,875
Mortgage-backed and asset-backed securities	94,560
Fixed Maturities, Available for Sale, Fair Value	$ 14,080,199	$ 14,493,060