Investments - Debt Securities by Maturity (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Less than one year	$ 1,199
One year through five years	6,997
After five years through ten years	4,150
Greater than ten years	4,021
Amortized Cost	20,349	$ 19,396
Fair Value
Less than one year	1,213
One year through five years	7,289
After five years through ten years	4,367
Greater than ten years	4,565
Total	21,504	21,188
Residential mortgage-backed securities
Amortized Cost
Debt securities, maturity, without single maturity date	744
Amortized Cost	744	673
Fair Value
Debt securities, maturity, without single maturity date	762
Total	762	705
Commercial mortgage-backed securities
Amortized Cost
Debt securities, maturity, without single maturity date	943
Amortized Cost	943	962
Fair Value
Debt securities, maturity, without single maturity date	985
Total	985	1,046
Other asset-backed securities
Amortized Cost
Debt securities, maturity, without single maturity date	2,295
Amortized Cost	2,295	2,369
Fair Value
Debt securities, maturity, without single maturity date	2,323
Total	$ 2,323	$ 2,403