Investments - Schedule of Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Amortized Cost
Due in one year or less	$ 45,343
Due from one to five years	586,187
Due from five to ten years	1,049,319
Due from ten to twenty years	3,939,736
Due after twenty years	7,786,358
Mortgage-backed and asset-backed securities	82,549
Fixed Maturities, Available for Sale, Amortized Cost	13,489,492	$ 13,251,871
Fair Value
Due in one year or less	45,953
Due from one to five years	631,214
Due from five to ten years	1,148,015
Due from ten to twenty years	4,356,812
Due after twenty years	8,190,986
Mortgage-backed and asset-backed securities	86,625
Fixed Maturities, Available for Sale, Fair Value	$ 14,459,605	$ 13,758,024