Fair Value Measurements - Fair Value of Financial Assets and Liabilities (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets		$ 169.7	$ 348.9
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets		2.6	0.0
Level 1 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		0.0	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross		0.0	0.0
Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		177.7	349.2
Financial liabilities
Derivative Liability, Fair Value, Gross		10.6	0.3
Level 3 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		0.0	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross		0.0	0.0
Netting Adjustments [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	[1]	(8.0)	(0.3)
Financial liabilities
Derivative Liability, Fair Value, Gross	[1]	(8.0)	(0.3)
Commodity Contract [Member] | Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		177.7	349.2
Financial liabilities
Derivative Liability, Fair Value, Gross		10.6	0.3
Long-term [Member] | Commodity Contract [Member]
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets		6.1	9.9
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets		1.6
Long-term [Member] | Commodity Contract [Member] | Level 1 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		0.0	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross		0.0
Long-term [Member] | Commodity Contract [Member] | Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		7.0	9.9
Financial liabilities
Derivative Liability, Fair Value, Gross		2.5	0.0
Long-term [Member] | Commodity Contract [Member] | Level 3 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		0.0	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross		0.0
Long-term [Member] | Commodity Contract [Member] | Netting Adjustments [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	[1]	(0.9)	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross	[1]	(0.9)
Short-term [Member] | Commodity Contract [Member]
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets		163.6	339.0
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets		1.0	0.0
Short-term [Member] | Commodity Contract [Member] | Level 1 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		0.0	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross		0.0	0.0
Short-term [Member] | Commodity Contract [Member] | Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		170.7	339.3
Financial liabilities
Derivative Liability, Fair Value, Gross		8.1	0.3
Short-term [Member] | Commodity Contract [Member] | Level 3 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		0.0	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross		0.0	0.0
Short-term [Member] | Commodity Contract [Member] | Netting Adjustments [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	[1]	(7.1)	(0.3)
Financial liabilities
Derivative Liability, Fair Value, Gross	[1]	$ (7.1)	$ (0.3)

[1] The Company nets its derivative contract assets and liabilities outstanding with the same counterparty on the Condensed Consolidated Balance Sheets, as the contracts contain netting provisions. Refer to Note 8 - Derivative Contracts, for additional information regarding the Company's derivative contracts.