Investments - Fair Values (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 21,041	$ 22,004
Gross unrealized, gains	843	1,355
Gross unrealized, loss	(317)	(98)
Fair value	21,567	23,261
U.S. government and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized cost	376	768
Gross unrealized, gains	27	38
Gross unrealized, loss	0	(2)
Fair value	403	804
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,036	2,001
Gross unrealized, gains	218	275
Gross unrealized, loss	(4)	(3)
Fair value	2,250	2,273
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized cost	17,731	18,262
Gross unrealized, gains	527	960
Gross unrealized, loss	(306)	(86)
Fair value	17,952	19,136
Foreign government
Debt Securities, Available-for-sale [Line Items]
Amortized cost	271	279
Gross unrealized, gains	12	20
Gross unrealized, loss	0	0
Fair value	283	299
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized cost	382	383
Gross unrealized, gains	4	6
Gross unrealized, loss	(4)	(4)
Fair value	382	385
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized cost	178	205
Gross unrealized, gains	48	49
Gross unrealized, loss	(1)	(1)
Fair value	225	253
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized cost	53	93
Gross unrealized, gains	6	6
Gross unrealized, loss	(2)	(2)
Fair value	57	97
Redeemable preferred stock
Debt Securities, Available-for-sale [Line Items]
Amortized cost	14	13
Gross unrealized, gains	1	1
Gross unrealized, loss	0	0
Fair value	$ 15	$ 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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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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