Investments - Schedule of Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Amortized Cost, net
Due in one year or less	$ 62,732
Due after one year through five years	745,715
Due after five years through ten years	1,783,317
Due after ten years through twenty years	5,788,845
Due after twenty years	8,097,882
Mortgage-backed and asset-backed securities	186,690
Fixed maturities, net of allowance for credit losses	16,665,181
Fair Value
Due in one year or less	63,550
Due after one year through five years	816,256
Due after five years through ten years	2,065,078
Due after ten years through twenty years	7,141,059
Due after twenty years	9,381,016
Mortgage-backed and asset-backed securities	194,223
Fixed maturities, fair value	$ 19,661,182	$ 18,907,147