Investments - Investments - Unrealized Losses on AFS Securities (Details) $ in Millions	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	781
Percentage of Gross Unrealized Losses Depressed Less than Twenty Percent of Cost or Amortized Cost	95.00%
ABS
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months Amortized Cost	$ 144	$ 566
Less Than 12 Months FV	143	566
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	0
12 Months or More Amortized Cost	11	113
12 Months or More FV	11	112
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	1
Total Amortized Cost	155	679
Total FV	154	678
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1	1
CLOs
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months Amortized Cost	1,109	1,358
Less Than 12 Months FV	1,105	1,338
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4	20
12 Months or More Amortized Cost	429	7
12 Months or More FV	425	7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4	0
Total Amortized Cost	1,538	1,365
Total FV	1,530	1,345
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	8	20
CMBS
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months Amortized Cost	88	896
Less Than 12 Months FV	87	882
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	14
12 Months or More Amortized Cost	31	1,129
12 Months or More FV	26	1,079
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5	50
Total Amortized Cost	119	2,025
Total FV	113	1,961
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	6	64
Corporate
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months Amortized Cost	930	7,174
Less Than 12 Months FV	918	6,903
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	12	271
12 Months or More Amortized Cost	497	2,541
12 Months or More FV	473	2,366
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	24	175
Total Amortized Cost	1,427	9,715
Total FV	1,391	9,269
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	36	446
Foreign Govt./Govt. Agencies
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months Amortized Cost	58	407
Less Than 12 Months FV	58	391
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	16
12 Months or More Amortized Cost	39	203
12 Months or More FV	38	193
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	10
Total Amortized Cost	97	610
Total FV	96	584
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1	26
Municipal
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months Amortized Cost	128	1,643
Less Than 12 Months FV	127	1,613
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	30
12 Months or More Amortized Cost	0	292
12 Months or More FV	0	275
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	17
Total Amortized Cost	128	1,935
Total FV	127	1,888
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1	47
RMBS
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months Amortized Cost	166	1,344
Less Than 12 Months FV	165	1,329
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	15
12 Months or More Amortized Cost	69	648
12 Months or More FV	69	628
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	20
Total Amortized Cost	235	1,992
Total FV	234	1,957
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1	35
U.S. Treasuries
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months Amortized Cost	37	497
Less Than 12 Months FV	37	492
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	5
12 Months or More Amortized Cost	119	339
12 Months or More FV	119	329
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	10
Total Amortized Cost	156	836
Total FV	156	821
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0	15
Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months Amortized Cost	2,660	13,885
Less Than 12 Months FV	2,640	13,514
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	20	371
12 Months or More Amortized Cost	1,195	5,272
12 Months or More FV	1,161	4,989
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	34	283
Total Amortized Cost	3,855	19,157
Total FV	3,801	18,503
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 54	$ 654