Investments - Investments by Category (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Current investments	$ 1,665	$ 2,045
Non-current investments	28,180	26,929
Total investments	29,845	28,974
Debt Securities [Member]
Schedule of Investments [Line Items]
Current investments	1,387	1,320
Non-current investments	22,474	21,608
Total investments	23,861	22,928
Equity securities [Member]
Schedule of Investments [Line Items]
Current investments	0	377
Non-current investments	245	171
Total investments	245	548
Commercial mortgage loans [Member]
Schedule of Investments [Line Items]
Current investments	21	32
Non-current investments	1,852	1,826
Total investments	1,873	1,858
Policy loans [Member]
Schedule of Investments [Line Items]
Current investments	0	0
Non-current investments	1,402	1,423
Total investments	1,402	1,423
Short-term investments [Member]
Schedule of Investments [Line Items]
Current investments	257	316
Non-current investments	0	0
Total investments	257	316
Other long-term investments [Member]
Schedule of Investments [Line Items]
Current investments	0	0
Non-current investments	2,207	1,901
Total investments	$ 2,207	$ 1,901