Fair Value of Financial Assets and Liabilities (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Financial Assets
Derivative instruments	$ 170.0	$ 348.9
Financial liabilities
Derivative instruments	4.8	0.0
Level 1 [Member]
Financial Assets
Derivative instruments	0.0	0.0
Financial liabilities
Derivative instruments	0.0	0.0
Level 2 [Member]
Financial Assets
Derivative instruments	171.0	349.2
Financial liabilities
Derivative instruments	5.8	0.3
Level 3 [Member]
Financial Assets
Derivative instruments	0.0	0.0
Financial liabilities
Derivative instruments	0.0	0.0
Netting Adjustments [Member]
Financial Assets
Derivative instruments	(1.0)	(0.3)
Financial liabilities
Derivative instruments	(1.0)	(0.3)
Long term [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments	23.2	9.9
Financial liabilities
Derivative instruments	4.0
Long term [Member] | Commodity Contract [Member] | Level 1 [Member]
Financial Assets
Derivative instruments	0.0	0.0
Long term [Member] | Commodity Contract [Member] | Level 2 [Member]
Financial Assets
Derivative instruments	23.2	9.9
Financial liabilities
Derivative instruments	4.0	0.0
Long term [Member] | Commodity Contract [Member] | Level 3 [Member]
Financial Assets
Derivative instruments	0.0	0.0
Long term [Member] | Commodity Contract [Member] | Netting Adjustments [Member]
Financial Assets
Derivative instruments	0.0	0.0
Financial liabilities
Derivative instruments	0.0
Short term [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments	146.8	339.0
Financial liabilities
Derivative instruments	0.8	0.0
Short term [Member] | Commodity Contract [Member] | Level 1 [Member]
Financial Assets
Derivative instruments	0.0	0.0
Financial liabilities
Derivative instruments	0.0	0.0
Short term [Member] | Commodity Contract [Member] | Level 2 [Member]
Financial Assets
Derivative instruments	147.8	339.3
Financial liabilities
Derivative instruments	1.8	0.3
Short term [Member] | Commodity Contract [Member] | Level 3 [Member]
Financial Assets
Derivative instruments	0.0	0.0
Financial liabilities
Derivative instruments	0.0	0.0
Short term [Member] | Commodity Contract [Member] | Netting Adjustments [Member]
Financial Assets
Derivative instruments	(1.0)	(0.3)
Financial liabilities
Derivative instruments	$ (1.0)	$ (0.3)