Derivative instruments - Fair Value of Derivative Contracts Falling under Early Termination Provisions in Net Liability Position (Details) $ in Millions	Dec. 31, 2016 USD ($)
A3 \| A-
Credit Derivatives [Line Items]
Potential close-out exposures (fair value)	$ 121
Baa2 \| BBB
Credit Derivatives [Line Items]
Potential close-out exposures (fair value)	805
Ba1 \| BB plus
Credit Derivatives [Line Items]
Potential close-out exposures (fair value)	$ 2,066

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624258-113959

Name:	us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant