Fair Value Measurements - Fair Value of Financial Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Financial Assets
Derivative Asset, Fair Value, Gross	$ 326.2		$ 349.2
Financial liabilities
Derivative Liability, Fair Value, Gross	0.8		0.3
Fair Value, Measurements, Recurring [Member]
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets	325.4		348.9
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets	0		0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	0		0
Financial liabilities
Derivative Liability, Fair Value, Gross	0		0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	326.2		349.2
Financial liabilities
Derivative Liability, Fair Value, Gross	0.8		0.3
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	0		0
Financial liabilities
Derivative Liability, Fair Value, Gross	0		0
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	(0.8)	[1]	(0.3)	[1]
Financial liabilities
Derivative Liability, Fair Value, Gross	(0.8)	[1]	(0.3)	[1]
Long-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member]
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets	13.7		9.9
Long-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	0		0
Long-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	13.7		9.9
Financial liabilities
Derivative Liability, Fair Value, Gross	0		0
Long-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	0		0
Long-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	0	[1]	0	[1]
Short-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member]
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets	311.7		339.0
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets	0		0
Short-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	0		0
Financial liabilities
Derivative Liability, Fair Value, Gross	0		0
Short-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	312.5		339.3
Financial liabilities
Derivative Liability, Fair Value, Gross	0.8		0.3
Short-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	0		0
Financial liabilities
Derivative Liability, Fair Value, Gross	0		0
Short-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	(0.8)	[1]	(0.3)	[1]
Financial liabilities
Derivative Liability, Fair Value, Gross	$ (0.8)	[1]	$ (0.3)	[1]

[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.