Investments - Schedule of Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fixed Maturities Available for Sale, Amortized Cost
Due in one year or less	$ 100,626
Due after one year through five years	643,842
Due after five years through ten years	1,670,278
Due after ten years through twenty years	5,246,650
Due after twenty years	8,346,465
Mortgage-backed and asset-backed securities	201,862
Total fixed maturities available for sale, Amortized Cost	16,209,723	$ 15,753,471
Fixed Maturities Available for Sale, Fair Value
Due in one year or less	102,237
Due after one year through five years	691,898
Due after five years through ten years	1,924,341
Due after ten years through twenty years	6,314,401
Due after twenty years	9,535,865
Mortgage-backed and asset-backed securities	228,932
Total fixed maturities available for sale, Fair Value	$ 18,797,674	$ 16,297,932