Investments in debt and equity securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Debt and Equity Securities
Cost Basis	$ 1,425	$ 1,317
Unrealized pretax net gains (losses)	14	50
Fair Value	(1,439)	(1,367)
U.S. treasury bonds
Schedule of Debt and Equity Securities
Cost Basis	10	9
Unrealized pretax net gains (losses)	0	0
Fair Value	(10)	(9)
Other U.S. and non-U.S. government bonds
Schedule of Debt and Equity Securities
Cost Basis	42	60
Unrealized pretax net gains (losses)	0	0
Fair Value	(42)	(60)
Corporate bonds
Schedule of Debt and Equity Securities
Cost Basis	585	489
Unrealized pretax net gains (losses)	(1)	3
Fair Value	(584)	(492)
Asset-backed securities
Schedule of Debt and Equity Securities
Cost Basis	67	90
Unrealized pretax net gains (losses)	0	0
Fair Value	(67)	(90)
U.S. governmental agency mortgage-backed securities
Schedule of Debt and Equity Securities
Cost Basis	265	225
Unrealized pretax net gains (losses)	(4)	(2)
Fair Value	(261)	(223)
Residential
Schedule of Debt and Equity Securities
Cost Basis	8	10
Unrealized pretax net gains (losses)	0	0
Fair Value	(8)	(10)
Commercial
Schedule of Debt and Equity Securities
Cost Basis	17	36
Unrealized pretax net gains (losses)	0	0
Fair Value	(17)	(36)
Large capitalization value
Schedule of Debt and Equity Securities
Cost Basis	287	280
Unrealized pretax net gains (losses)	(3)	32
Fair Value	(284)	(312)
REIT
Schedule of Debt and Equity Securities
Cost Basis	104	77
Unrealized pretax net gains (losses)	6	2
Fair Value	(110)	(79)
Smaller company growth
Schedule of Debt and Equity Securities
Cost Basis	40	41
Unrealized pretax net gains (losses)	16	15
Fair Value	$ (56)	$ (56)

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-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
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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+ References

Reference 1: 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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