Impairment fair value, ceiling test and transactions costs (Details 4) - Discontinued operations, held-for-sale or disposed by sale [Member] - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Fair value impairment	$ (1.6)	$ 356.1	$ 400.0
Fair value impairment after tax	(1.0)	$ 224.4	$ 252.0
Accrued liability of estimated transaction costs	2.5				$ 2.5
Expensed exit and disposal costs					4.9
Estimated future exit and disposal costs	6.1				6.1
Future lease payments	4.2				$ 4.2
Lease termination payment	$ 3.3
Impairment of oil and gas properties				$ 500.4
Impairment of oil and gas properties, after tax				$ 315.3
Acquisition costs						$ 209.2