Investments - Schedule of Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost, net
Due in one year or less	$ 46,963
Due after one year through five years	762,252
Due after five years through ten years	1,794,849
Due after ten years through twenty years	6,027,005
Due after twenty years	8,067,809
Mortgage-backed and asset-backed securities	188,789
Fixed maturities, net of allowance for credit losses	16,887,667
Fair Value
Due in one year or less	47,877
Due after one year through five years	836,895
Due after five years through ten years	2,118,729
Due after ten years through twenty years	7,578,897
Due after twenty years	9,496,378
Mortgage-backed and asset-backed securities	198,280
Fixed maturities, fair value	$ 20,277,056	$ 18,907,147