Investments - Amortized cost, gross unrealized gains and losses and fair value for fixed income securities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available for Sale Securities
Amortized cost	$ 56,209	$ 57,525
Gross unrealized gains	1,210	1,750
Gross unrealized losses	(745)	(283)
Fair value	56,674	58,992
U.S. government and agencies
Schedule of Available for Sale Securities
Amortized cost	3,373	3,580
Gross unrealized gains	50	56
Gross unrealized losses	(17)	(20)
Fair value	3,406	3,616
Municipal
Schedule of Available for Sale Securities
Amortized cost	8,404	8,053
Gross unrealized gains	257	311
Gross unrealized losses	(92)	(36)
Fair value	8,569	8,328
Corporate
Schedule of Available for Sale Securities
Amortized cost	41,699	42,996
Gross unrealized gains	763	1,234
Gross unrealized losses	(611)	(204)
Fair value	41,851	44,026
Foreign government
Schedule of Available for Sale Securities
Amortized cost	968	1,005
Gross unrealized gains	21	27
Gross unrealized losses	(10)	(11)
Fair value	979	1,021
Asset-backed securities (“ABS”)
Schedule of Available for Sale Securities
Amortized cost	1,196	1,266
Gross unrealized gains	11	13
Gross unrealized losses	(10)	(7)
Fair value	1,197	1,272
Residential mortgage-backed securities (“RMBS”)
Schedule of Available for Sale Securities
Amortized cost	453	480
Gross unrealized gains	100	101
Gross unrealized losses	(3)	(3)
Fair value	550	578
Commercial mortgage-backed securities (“CMBS”)
Schedule of Available for Sale Securities
Amortized cost	95	124
Gross unrealized gains	6	6
Gross unrealized losses	(2)	(2)
Fair value	99	128
Redeemable preferred stock
Schedule of Available for Sale Securities
Amortized cost	21	21
Gross unrealized gains	2	2
Gross unrealized losses	0	0
Fair value	$ 23	$ 23

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
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
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-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Topic 320
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-Publisher FASB
-Name Accounting Standards Codification
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