Investments - Investments - Unrealized Losses on AFS Securities (Details) $ in Millions	Sep. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	$ 16,755	$ 6,798
Amortized Cost
12 Months or More	3,183	2,040
Total	19,938	8,838
Fair Value
Less Than 12 Months	16,348	6,739
12 Months or More	2,983	1,969
Total	19,331	8,708
Unrealized Losses
Less Than 12 Months	(407)	(59)
12 Months or More	(200)	(71)
Total	$ (607)	(130)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	2,926
Percentage of Gross Unrealized Losses Depressed Less than Twenty Percent of Cost or Amortized Cost	98.53042%
ABS
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	$ 668	461
Amortized Cost
12 Months or More	74	30
Total	742	491
Fair Value
Less Than 12 Months	666	460
12 Months or More	73	29
Total	739	489
Unrealized Losses
Less Than 12 Months	(2)	(1)
12 Months or More	(1)	(1)
Total	(3)	(2)
CLOs
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	929	359
Amortized Cost
12 Months or More	10	1
Total	939	360
Fair Value
Less Than 12 Months	926	359
12 Months or More	10	1
Total	936	360
Unrealized Losses
Less Than 12 Months	(3)	0
12 Months or More	0	0
Total	(3)	0
CMBS
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	1,995	1,178
Amortized Cost
12 Months or More	740	243
Total	2,735	1,421
Fair Value
Less Than 12 Months	1,952	1,167
12 Months or More	697	228
Total	2,649	1,395
Unrealized Losses
Less Than 12 Months	(43)	(11)
12 Months or More	(43)	(15)
Total	(86)	(26)
Corporate
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	7,586	2,322
Amortized Cost
12 Months or More	1,412	1,064
Total	8,998	3,386
Fair Value
Less Than 12 Months	7,364	2,302
12 Months or More	1,310	1,028
Total	8,674	3,330
Unrealized Losses
Less Than 12 Months	(222)	(20)
12 Months or More	(102)	(36)
Total	(324)	(56)
Foreign Govt./Govt. Agencies
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	552	244
Amortized Cost
12 Months or More	95	51
Total	647	295
Fair Value
Less Than 12 Months	537	242
12 Months or More	90	49
Total	627	291
Unrealized Losses
Less Than 12 Months	(15)	(2)
12 Months or More	(5)	(2)
Total	(20)	(4)
Municipal
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	2,206	511
Amortized Cost
12 Months or More	266	236
Total	2,472	747
Fair Value
Less Than 12 Months	2,151	507
12 Months or More	245	228
Total	2,396	735
Unrealized Losses
Less Than 12 Months	(55)	(4)
12 Months or More	(21)	(8)
Total	(76)	(12)
RMBS
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	1,795	889
Amortized Cost
12 Months or More	184	137
Total	1,979	1,026
Fair Value
Less Than 12 Months	1,751	887
12 Months or More	176	135
Total	1,927	1,022
Unrealized Losses
Less Than 12 Months	(44)	(2)
12 Months or More	(8)	(2)
Total	(52)	(4)
U.S. Treasuries
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	1,024	658
Amortized Cost
12 Months or More	402	254
Total	1,426	912
Fair Value
Less Than 12 Months	1,001	652
12 Months or More	382	250
Total	1,383	902
Unrealized Losses
Less Than 12 Months	(23)	(6)
12 Months or More	(20)	(4)
Total	$ (43)	(10)
Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost		6,622
Amortized Cost
12 Months or More		2,016
Total		8,638
Fair Value
Less Than 12 Months		6,576
12 Months or More		1,948
Total		8,524
Unrealized Losses
Less Than 12 Months		(46)
12 Months or More		(68)
Total		(114)
Equity Securities, at fair value
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost		176
Amortized Cost
12 Months or More		24
Total		200
Fair Value
Less Than 12 Months		163
12 Months or More		21
Total		184
Unrealized Losses
Less Than 12 Months		(13)
12 Months or More		(3)
Total		$ (16)