Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 64.4	$ 104.4
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(13.5)	(23.5)
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	50.9	80.9
Oil derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	40.8	84.0
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(27.9)	(33.1)
Natural gas derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	21.5	17.9
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	15.4	11.4
Natural gas derivative contracts [Member] | Storage [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	2.1	2.5
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ (1.0)	$ (1.8)