Impairment fair value and ceiling test (Details 2) - Oil and natural gas properties [Member] - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percent of oil and natural gas production to be sold				90.00%
Discontinued operation, provision for loss (gain) on disposal, before income tax	$ 356.1	$ 400.0
Discontinued operation, provision for loss (gain) on disposal, net of tax	224.4	$ 252.0
Disposal group - estimated transaction costs	3.0			$ 3.0
Disposal group - exit and disposal costs	11.0			11.0
Disposal group - potential future lease payments	$ 12.0			$ 12.0
Impairment of oil and gas properties			$ 500.4
Impairment of oil and gas properties, after tax			$ 315.3