Fair Value of Financial Assets and Liabilities (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Financial Assets
Derivative instruments	$ 0.0	$ 170.0
Financial liabilities
Derivative instruments	201.8	4.8
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	0.0	171.0
Financial liabilities
Derivative instruments	201.8	5.8
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	0.0	(1.0)
Financial liabilities
Derivative instruments	0.0	(1.0)
Long term [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments	0.0	23.2
Financial liabilities
Derivative instruments	32.0	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	0.0	23.2
Financial liabilities
Derivative instruments	32.0	4.0
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	0.0
Short term [Member] | Commodity Contract [Member]
Financial Assets
Derivative instruments	0.0	146.8
Financial liabilities
Derivative instruments	169.8	0.8
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	0.0	147.8
Financial liabilities
Derivative instruments	169.8	1.8
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	0.0	(1.0)
Financial liabilities
Derivative instruments	$ 0.0	$ (1.0)