Investments - Investments - Unrealized Losses on AFS Securities (Details) $ in Millions	Jun. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	$ 15,515	$ 6,798
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	2,855
Amortized Cost
12 Months or More	$ 1,981	2,040
Total	17,496	8,838
Fair Value
Less Than 12 Months	15,101	6,739
12 Months or More	1,845	1,969
Total	16,946	8,708
Unrealized Losses
Less Than 12 Months	(414)	(59)
12 Months or More	(136)	(71)
Total	$ (550)	(130)
Percentage of Gross Unrealized Losses Depressed Less than Twenty Percent of Cost or Amortized Cost	98.42382%
ABS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	$ 477	461
Amortized Cost
12 Months or More	32	30
Total	509	491
Fair Value
Less Than 12 Months	475	460
12 Months or More	31	29
Total	506	489
Unrealized Losses
Less Than 12 Months	(2)	(1)
12 Months or More	(1)	(1)
Total	(3)	(2)
CDOs
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	556	359
Amortized Cost
12 Months or More	0	1
Total	556	360
Fair Value
Less Than 12 Months	553	359
12 Months or More	0	1
Total	553	360
Unrealized Losses
Less Than 12 Months	(3)	0
12 Months or More	0	0
Total	(3)	0
CMBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	2,002	1,178
Amortized Cost
12 Months or More	251	243
Total	2,253	1,421
Fair Value
Less Than 12 Months	1,947	1,167
12 Months or More	233	228
Total	2,180	1,395
Unrealized Losses
Less Than 12 Months	(55)	(11)
12 Months or More	(18)	(15)
Total	(73)	(26)
Corporate
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	7,467	2,322
Amortized Cost
12 Months or More	1,035	1,064
Total	8,502	3,386
Fair Value
Less Than 12 Months	7,213	2,302
12 Months or More	953	1,028
Total	8,166	3,330
Unrealized Losses
Less Than 12 Months	(254)	(20)
12 Months or More	(82)	(36)
Total	(336)	(56)
Foreign Govt./Govt. Agencies
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	676	244
Amortized Cost
12 Months or More	61	51
Total	737	295
Fair Value
Less Than 12 Months	652	242
12 Months or More	57	49
Total	709	291
Unrealized Losses
Less Than 12 Months	(24)	(2)
12 Months or More	(4)	(2)
Total	(28)	(4)
Municipal
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	1,845	511
Amortized Cost
12 Months or More	225	236
Total	2,070	747
Fair Value
Less Than 12 Months	1,817	507
12 Months or More	209	228
Total	2,026	735
Unrealized Losses
Less Than 12 Months	(28)	(4)
12 Months or More	(16)	(8)
Total	(44)	(12)
RMBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	1,751	889
Amortized Cost
12 Months or More	137	137
Total	1,888	1,026
Fair Value
Less Than 12 Months	1,716	887
12 Months or More	131	135
Total	1,847	1,022
Unrealized Losses
Less Than 12 Months	(35)	(2)
12 Months or More	(6)	(2)
Total	(41)	(4)
U.S. Treasuries
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Amortized Cost	741	658
Amortized Cost
12 Months or More	240	254
Total	981	912
Fair Value
Less Than 12 Months	728	652
12 Months or More	231	250
Total	959	902
Unrealized Losses
Less Than 12 Months	(13)	(6)
12 Months or More	(9)	(4)
Total	$ (22)	(10)
Debt Securities [Member]
Schedule of Available-for-sale Securities [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
Schedule of Available-for-sale Securities [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)