Investments - Fair Values (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 20,512	$ 21,057
Gross unrealized, gains	1,386	732
Gross unrealized, loss	(55)	(389)
Fair value	21,843	21,400
U.S. government and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized cost	558	740
Gross unrealized, gains	39	33
Gross unrealized, loss	0	0
Fair value	597	773
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,757	1,997
Gross unrealized, gains	275	202
Gross unrealized, loss	(3)	(4)
Fair value	2,029	2,195
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized cost	17,520	17,521
Gross unrealized, gains	1,001	433
Gross unrealized, loss	(46)	(381)
Fair value	18,475	17,573
Foreign government
Debt Securities, Available-for-sale [Line Items]
Amortized cost	155	170
Gross unrealized, gains	9	9
Gross unrealized, loss	0	0
Fair value	164	179
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized cost	347	429
Gross unrealized, gains	7	3
Gross unrealized, loss	(3)	(3)
Fair value	351	429
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized cost	133	154
Gross unrealized, gains	47	44
Gross unrealized, loss	0	(1)
Fair value	180	197
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized cost	29	33
Gross unrealized, gains	7	7
Gross unrealized, loss	(3)	0
Fair value	33	40
Redeemable preferred stock
Debt Securities, Available-for-sale [Line Items]
Amortized cost	13	13
Gross unrealized, gains	1	1
Gross unrealized, loss	0	0
Fair value	$ 14	$ 14

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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