Investments - Schedule of Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fixed Maturities Available for Sale, Amortized Cost
Due in one year or less	$ 151,815
Due after one year through five years	631,205
Due after five years through ten years	1,646,091
Due after ten years through twenty years	5,211,263
Due after twenty years	8,140,905
Mortgage-backed and asset-backed securities	202,993
Total fixed maturities available for sale, Amortized Cost	15,984,272	$ 15,753,471
Fixed Maturities Available for Sale, Fair Value
Due in one year or less	153,027
Due after one year through five years	674,283
Due after five years through ten years	1,867,864
Due after ten years through twenty years	6,086,465
Due after twenty years	8,922,971
Mortgage-backed and asset-backed securities	229,664
Total fixed maturities available for sale, Fair Value	$ 17,934,274	$ 16,297,932