Investments- Fair Values (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 22,324	$ 23,112
Gross unrealized gains	1,429	1,310
Gross unrealized losses	(102)	(200)
Fair value	23,651	24,222
U.S. government and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	609	968
Gross unrealized gains	45	48
Gross unrealized losses	(1)	(2)
Fair value	653	1,014
Municipal
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,000	2,017
Gross unrealized gains	275	264
Gross unrealized losses	(5)	(7)
Fair value	2,270	2,274
Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	18,639	18,945
Gross unrealized gains	1,015	905
Gross unrealized losses	(80)	(169)
Fair value	19,574	19,681
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	286	304
Gross unrealized gains	26	28
Gross unrealized losses	0	0
Fair value	312	332
ABS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	405	337
Gross unrealized gains	6	4
Gross unrealized losses	(8)	(10)
Fair value	403	331
RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	239	294
Gross unrealized gains	47	42
Gross unrealized losses	(1)	(3)
Fair value	285	333
CMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	133	233
Gross unrealized gains	13	17
Gross unrealized losses	(7)	(9)
Fair value	139	241
Redeemable preferred stock
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	13	14
Gross unrealized gains	2	2
Gross unrealized losses	0	0
Fair value	$ 15	$ 16