Investments- Fair Values (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 22,532	$ 23,112
Gross unrealized gains	1,330	1,310
Gross unrealized losses	(166)	(200)
Fair value	23,696	24,222
U.S. government and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	589	968
Gross unrealized gains	45	48
Gross unrealized losses	0	(2)
Fair value	634	1,014
Municipal
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,011	2,017
Gross unrealized gains	261	264
Gross unrealized losses	(6)	(7)
Fair value	2,266	2,274
Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	18,779	18,945
Gross unrealized gains	932	905
Gross unrealized losses	(139)	(169)
Fair value	19,572	19,681
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	300	304
Gross unrealized gains	27	28
Gross unrealized losses	0	0
Fair value	327	332
ABS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	406	337
Gross unrealized gains	5	4
Gross unrealized losses	(9)	(10)
Fair value	402	331
RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	260	294
Gross unrealized gains	44	42
Gross unrealized losses	(3)	(3)
Fair value	301	333
CMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	174	233
Gross unrealized gains	14	17
Gross unrealized losses	(9)	(9)
Fair value	179	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