Income taxes (Income Before Income Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Examination [Line Items]
Income before income taxes	$ 9,597	$ 9,163	$ 7,978
Domestic
Income Tax Examination [Line Items]
Income before income taxes	4,436	4,478	3,532
Foreign
Income Tax Examination [Line Items]
Income before income taxes	$ 5,161	$ 4,685	$ 4,446

Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h)(1)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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