Investments - Fair Values (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 21,645	$ 22,004
Gross unrealized gains	1,008	1,355
Gross unrealized losses	(252)	(98)
Fair value	22,401	23,261
U.S. government and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	538	768
Gross unrealized gains	31	38
Gross unrealized losses	(1)	(2)
Fair value	568	804
Municipal
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,033	2,001
Gross unrealized gains	234	275
Gross unrealized losses	(4)	(3)
Fair value	2,263	2,273
Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	18,135	18,262
Gross unrealized gains	667	960
Gross unrealized losses	(239)	(86)
Fair value	18,563	19,136
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	271	279
Gross unrealized gains	15	20
Gross unrealized losses	0	0
Fair value	286	299
ABS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	399	383
Gross unrealized gains	6	6
Gross unrealized losses	(5)	(4)
Fair value	400	385
RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	191	205
Gross unrealized gains	49	49
Gross unrealized losses	(1)	(1)
Fair value	239	253
CMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	64	93
Gross unrealized gains	5	6
Gross unrealized losses	(2)	(2)
Fair value	67	97
Redeemable preferred stock
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	14	13
Gross unrealized gains	1	1
Gross unrealized losses	0	0
Fair value	$ 15	$ 14