Commitments and contingencies (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Maturity schedule for contractual commitments [Abstract]
Assets pledged under mortgages and overdraft facilities	$ 5,367,700,000	$ 4,953,400,000
West Capricorn
Maturity schedule for contractual commitments [Abstract]
Contractual commitments	$ 0	$ 0

The value of assets other than financial instruments pledged to provide security to a suppler under a recorded unconditional purchase obligation arrangement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=34720971&loc=SL6224234-111729

Name:	us-gaap_RecordedUnconditionalPurchaseObligationNonfinancialAssetsPledgedAsSecurityValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant