Investments - Schedule of Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost, net
Due in one year or less	$ 70,732
Due after one year through five years	781,183
Due after five years through ten years	1,870,527
Due after ten years through twenty years	6,109,815
Due after twenty years	8,169,528
Mortgage-backed and asset-backed securities	192,014
Fixed maturities, net of allowance for credit losses	17,193,799
Fair Value
Due in one year or less	72,395
Due after one year through five years	866,408
Due after five years through ten years	2,260,011
Due after ten years through twenty years	7,884,526
Due after twenty years	9,923,706
Mortgage-backed and asset-backed securities	206,463
Fixed maturities, fair value	$ 21,213,509	$ 18,907,147