Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed Maturities [Member] - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	$ 16,827	$ 3,979
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,356	84
Estimated Fair Value - 12 months or greater	805	1,238
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	105	26
Estimated Fair Value	17,632	5,217
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	1,461	110
Non-investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	1,294	220
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	244	38
Estimated Fair Value - 12 months or greater	73	110
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	13	8
Estimated Fair Value	1,367	330
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	257	46
Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	15,533	3,759
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,112	46
Estimated Fair Value - 12 months or greater	732	1,128
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	92	18
Estimated Fair Value	16,265	4,887
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	1,204	64
Corporate Debt Securities [Member] | Non-investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	1,106	220
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	219	38
Estimated Fair Value - 12 months or greater	66	100
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	11	7
Estimated Fair Value	1,172	320
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	230	45
Corporate Debt Securities [Member] | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	9,666	1,936
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	729	29
Estimated Fair Value - 12 months or greater	93	293
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	18	7
Estimated Fair Value	9,759	2,229
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	747	36
Canadian and Canadian provincial governments | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Estimated Fair Value - 12 months or greater	6	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	0
Estimated Fair Value	6	0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	1	0
Residential mortgage-backed securities | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	1,047	367
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	25	2
Estimated Fair Value - 12 months or greater	41	84
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2	1
Estimated Fair Value	1,088	451
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	27	3
Asset-backed Securities [Member] | Non-investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	15	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9	0
Estimated Fair Value - 12 months or greater	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Estimated Fair Value	15	0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	9	0
Asset-backed Securities [Member] | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	1,936	773
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	203	5
Estimated Fair Value - 12 months or greater	552	739
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	65	9
Estimated Fair Value	2,488	1,512
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	268	14
Commercial mortgage-backed securities | Non-investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	23	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3
Estimated Fair Value - 12 months or greater	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Estimated Fair Value	23	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	3
Commercial mortgage-backed securities | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	913	253
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	52	3
Estimated Fair Value - 12 months or greater	21	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	0
Estimated Fair Value	934	253
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	53	3
US Government Agencies Debt Securities [Member] | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	0	49
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	1
Estimated Fair Value - 12 months or greater	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Estimated Fair Value	0	49
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	0	1
US States and Political Subdivisions Debt Securities [Member] | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	158	103
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9	2
Estimated Fair Value - 12 months or greater	12	12
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	1
Estimated Fair Value	170	115
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	10	3
Debt Security, Government, Non-US [Member] | Non-investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	150	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	13	0
Estimated Fair Value - 12 months or greater	7	10
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2	1
Estimated Fair Value	157	10
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	15	1
Debt Security, Government, Non-US [Member] | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	1,813	278
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	94	4
Estimated Fair Value - 12 months or greater	7	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4	0
Estimated Fair Value	1,820	278
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	$ 98	$ 4