Investments - Schedule of Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Amortized Cost, net
Due in one year or less	$ 51,897
Due after one year through five years	668,783
Due after five years through ten years	1,684,359
Due after ten years through twenty years	5,647,201
Due after twenty years	8,099,163
Mortgage-backed and asset-backed securities	188,687
Fixed maturities, net of allowance for credit losses	16,340,090
Fair Value
Due in one year or less	52,538
Due after one year through five years	705,821
Due after five years through ten years	1,852,455
Due after ten years through twenty years	6,462,058
Due after twenty years	8,613,350
Mortgage-backed and asset-backed securities	193,319
Fixed maturities, fair value	$ 17,879,541	$ 18,907,147