Investments - Investments by Category (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Current investments	$ 2,045	$ 2,136
Non-current investments	26,929	26,483
Total investments	28,974	28,619
Fixed maturities [Member]
Schedule of Investments [Line Items]
Current investments	1,320	1,516
Non-current investments	21,608	21,622
Total investments	22,928	23,138
Equity securities [Member]
Schedule of Investments [Line Items]
Current investments	377	406
Non-current investments	171	182
Total investments	548	588
Commercial mortgage loans [Member]
Schedule of Investments [Line Items]
Current investments	32	15
Non-current investments	1,826	1,746
Total investments	1,858	1,761
Policy loans [Member]
Schedule of Investments [Line Items]
Current investments	0	0
Non-current investments	1,423	1,415
Total investments	1,423	1,415
Short-term investments [Member]
Schedule of Investments [Line Items]
Current investments	316	199
Non-current investments	0	0
Total investments	316	199
Other long-term investments [Member]
Schedule of Investments [Line Items]
Current investments	0	0
Non-current investments	1,901	1,518
Total investments	$ 1,901	$ 1,518