Investments - Investments by Category (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Current	$ 1,374	$ 1,331
Long-term	23,756	23,262
Total	25,130	24,593
Debt securities
Schedule of Investments [Line Items]
Current	895	959
Long-term	16,962	17,172
Total	17,857	18,131
Equity securities
Schedule of Investments [Line Items]
Current	0	0
Long-term	551	501
Total	551	501
Commercial mortgage loans
Schedule of Investments [Line Items]
Current	40	13
Long-term	1,485	1,406
Total	1,525	1,419
Policy loans
Schedule of Investments [Line Items]
Current	0	0
Long-term	1,329	1,351
Total	1,329	1,351
Other long-term investments
Schedule of Investments [Line Items]
Current	0	0
Long-term	3,429	2,832
Total	3,429	2,832
Short-term investments
Schedule of Investments [Line Items]
Current	439	359
Long-term	0	0
Total	$ 439	$ 359