Fair Value Measurements Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Impairment of Oil and Gas Properties		$ 1,182.4
Fair Value, Measurements, Recurring [Member]
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets		155.2		$ 170.0
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets		3.4		4.8
Fair Value, Measurements, Recurring [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
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		155.4		171.0
Financial liabilities
Derivative Liability, Fair Value, Gross		3.6		5.8
Fair Value, Measurements, Recurring [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
Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	[1]	(0.2)		(1.0)
Financial liabilities
Derivative Liability, Fair Value, Gross	[1]	(0.2)		(1.0)
Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		155.4		171.0
Financial liabilities
Derivative Liability, Fair Value, Gross		3.6		5.8
Long-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member]
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets		20.8		23.2
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets		3.4		4.0
Long-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [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] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		20.9		23.2
Financial liabilities
Derivative Liability, Fair Value, Gross		3.5		4.0
Long-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [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] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	[1]	(0.1)		0.0
Financial liabilities
Derivative Liability, Fair Value, Gross	[1]	(0.1)		0.0
Short-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member]
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets		134.4		146.8
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets		0.0		0.8
Short-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [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] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		134.5		147.8
Financial liabilities
Derivative Liability, Fair Value, Gross		0.1		1.8
Short-term [Member] | Commodity Contract [Member] | Fair Value, Measurements, Recurring [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] | Fair Value, Measurements, Recurring [Member] | Netting Adjustments [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	[1]	(0.1)		(1.0)
Financial liabilities
Derivative Liability, Fair Value, Gross	[1]	(0.1)		$ (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.