Investments - Amortized cost, gross unrealized gains and losses and fair value for fixed income securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available for Sale Securities
Amortized cost	$ 57,525	$ 56,576
Gross unrealized gains	1,750	1,710
Gross unrealized losses	(283)	(447)
Fixed income securities	58,992	57,839
U.S. government and agencies
Schedule of Available for Sale Securities
Amortized cost	3,580	3,572
Gross unrealized gains	56	74
Gross unrealized losses	(20)	(9)
Fixed income securities	3,616	3,637
Municipal
Schedule of Available for Sale Securities
Amortized cost	8,053	7,116
Gross unrealized gains	311	304
Gross unrealized losses	(36)	(87)
Fixed income securities	8,328	7,333
Corporate
Schedule of Available for Sale Securities
Amortized cost	42,996	42,742
Gross unrealized gains	1,234	1,178
Gross unrealized losses	(204)	(319)
Fixed income securities	44,026	43,601
Foreign government
Schedule of Available for Sale Securities
Amortized cost	1,005	1,043
Gross unrealized gains	27	36
Gross unrealized losses	(11)	(4)
Fixed income securities	1,021	1,075
ABS
Schedule of Available for Sale Securities
Amortized cost	1,266	1,169
Gross unrealized gains	13	13
Gross unrealized losses	(7)	(11)
Fixed income securities	1,272	1,171
RMBS
Schedule of Available for Sale Securities
Amortized cost	480	651
Gross unrealized gains	101	85
Gross unrealized losses	(3)	(8)
Fixed income securities	578	728
CMBS
Schedule of Available for Sale Securities
Amortized cost	124	262
Gross unrealized gains	6	17
Gross unrealized losses	(2)	(9)
Fixed income securities	128	270
Redeemable preferred stock
Schedule of Available for Sale Securities
Amortized cost	21	21
Gross unrealized gains	2	3
Gross unrealized losses	0	0
Fixed income securities	$ 23	$ 24

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
-Section 25
-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
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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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-Publisher FASB
-Name Accounting Standards Codification
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