Fair value option - Changes in Fair Value of the Loans and Long-Term Debt and the Location of the Changes (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment and other income \| Loans Receivable
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in the fair value	$ 12	$ 3	$ 0
Foreign exchange and other trading revenue (losses) \| Long-term debt
Fair Value, Option, Quantitative Disclosures [Line Items]
Changes in the fair value	$ (4)	$ (12)	$ (26)

For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14172-108612

Name:	us-gaap_FairValueOptionChangesInFairValueGainLoss1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration