Investments- Fair Values (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 22,081	$ 23,112
Gross unrealized gains	1,431	1,310
Gross unrealized losses	(87)	(200)
Fair value	23,425	24,222
U.S. government and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	569	968
Gross unrealized gains	43	48
Gross unrealized losses	(1)	(2)
Fair value	611	1,014
Municipal
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,985	2,017
Gross unrealized gains	272	264
Gross unrealized losses	(5)	(7)
Fair value	2,252	2,274
Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	18,516	18,945
Gross unrealized gains	1,027	905
Gross unrealized losses	(69)	(169)
Fair value	19,474	19,681
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	286	304
Gross unrealized gains	24	28
Gross unrealized losses	0	0
Fair value	310	332
ABS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	373	337
Gross unrealized gains	6	4
Gross unrealized losses	(7)	(10)
Fair value	372	331
RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	221	294
Gross unrealized gains	49	42
Gross unrealized losses	(1)	(3)
Fair value	269	333
CMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	118	233
Gross unrealized gains	8	17
Gross unrealized losses	(4)	(9)
Fair value	122	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

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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