Derivative instruments gains and losses on derivative instruments (Details 8) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Commodity derivatives [Member] \| Not designated as hedging instrument [Member] \| Exploration and production [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount of gain (loss) recognized in discontinued operations, before tax	$ (18.3)	$ 23.4

Amount of gain (loss) recognized in earnings in the period from the increase (decrease) in fair value of price risk derivatives not designated as hedging instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959

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