Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) derivative contracts not designated as hedging instruments	$ 134.0	$ 460.9	$ (11.1)
Unrealized gain (loss) derivative contracts not designated as hedging instruments	(367.0)	(183.7)	374.4
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(233.0)	277.2	363.3
Realized and unrealized gains on commodity derivative instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	134.0	460.9	(3.5)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(367.0)	(183.7)	372.4
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	(233.0)	277.2	368.9
Interest Rate Swap [Member] | Realized and unrealized gains losses on interest rate swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	0.0	(7.6)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	0.0	2.0
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	0.0	0.0	(5.6)
Oil derivative contracts [Member] | Production [Member] | Realized and unrealized gains on commodity derivative instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	86.3	353.7	15.7
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(217.2)	(244.9)	299.8
Gas derivative contracts [Member] | Production [Member] | Realized and unrealized gains on commodity derivative instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	44.8	103.4	(16.7)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(145.4)	62.0	68.4
Gas derivative contracts [Member] | Storage [Member] | Realized and unrealized gains on commodity derivative instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	2.9	3.8	(2.5)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ (4.4)	$ (0.8)	$ 4.2