Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 6.4	$ 49.5	$ (10.0)	$ 113.9
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	100.3	(230.0)	277.6	(243.5)
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	106.7	(180.5)	267.6	(129.6)
Oil derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	11.5	19.9	9.5	60.7
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	70.5	(107.7)	174.8	(135.6)
Natural gas derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	(5.1)	28.9	(19.3)	50.4
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	29.4	(120.2)	100.5	(104.8)
Natural gas derivative contracts [Member] | Storage [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	0.7	(0.2)	2.8
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 0.4	$ (2.1)	$ 2.3	$ (3.1)