Fair Value Measurements Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets		$ 1.4		$ 170.0
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets		79.7		4.8
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		8.3		171.0
Financial liabilities
Derivative Liability, Fair Value, Gross		86.6		5.8
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]	(6.9)		(1.0)
Financial liabilities
Derivative Liability, Fair Value, Gross	[1]	(6.9)		(1.0)
Long-term [Member]
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets		0.0		23.2
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets		33.1		4.0
Long-term [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
Long-term [Member] | Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		0.6		23.2
Financial liabilities
Derivative Liability, Fair Value, Gross		33.7		4.0
Long-term [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
Long-term [Member] | Netting Adjustments [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	[1]	(0.6)		0.0
Financial liabilities
Derivative Liability, Fair Value, Gross	[1]	(0.6)		0.0
Short-term [Member]
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets		1.4		146.8
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets		46.6		0.8
Short-term [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] | Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		7.7		147.8
Financial liabilities
Derivative Liability, Fair Value, Gross		52.9		1.8
Short-term [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] | Netting Adjustments [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	[1]	(6.3)		(1.0)
Financial liabilities
Derivative Liability, Fair Value, Gross	[1]	(6.3)		$ (1.0)
Proved Property [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Impairment of Oil and Gas Properties		$ 1,167.9	$ 19.4

[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 7 – Derivative Contracts, for additional information regarding the Company's derivative contracts.