Investments - Fair Values (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 20,664	$ 21,057
Gross unrealized, gains	1,006	732
Gross unrealized, loss	(129)	(389)
Fair value	21,541	21,400
U.S. government and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized cost	504	740
Gross unrealized, gains	34	33
Gross unrealized, loss	0	0
Fair value	538	773
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,927	1,997
Gross unrealized, gains	236	202
Gross unrealized, loss	(3)	(4)
Fair value	2,160	2,195
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized cost	17,465	17,521
Gross unrealized, gains	673	433
Gross unrealized, loss	(122)	(381)
Fair value	18,016	17,573
Foreign government
Debt Securities, Available-for-sale [Line Items]
Amortized cost	170	170
Gross unrealized, gains	8	9
Gross unrealized, loss	0	0
Fair value	178	179
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized cost	410	429
Gross unrealized, gains	3	3
Gross unrealized, loss	(3)	(3)
Fair value	410	429
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized cost	144	154
Gross unrealized, gains	44	44
Gross unrealized, loss	0	(1)
Fair value	188	197
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized cost	31	33
Gross unrealized, gains	7	7
Gross unrealized, loss	(1)	0
Fair value	37	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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Amortized cost of 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 of 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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