Fair Value Measurements Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2017	Dec. 31, 2016
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets			$ 21.1	$ 0.0
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets			45.6	201.8
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			29.3	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross			53.8	201.8
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]		(8.2)	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross	[1]		(8.2)	0.0
Long-term [Member]
Financial Assets
Net amount of derivative assets presented on condensed consolidated balance sheets			21.1	0.0
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets			0.3	32.0
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			21.7	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross			0.9	32.0
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			0.0	0.0
Financial liabilities
Net amount of derivative liabilities presented on condensed consolidated balance sheets			45.3	169.8
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			7.6	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross			52.9	169.8
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]		(7.6)	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross	[1]		$ (7.6)	$ 0.0
Proved Property [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Impairment of Oil and Gas Properties		$ 1,167.9

[1] The Company nets its derivative contract assets and liabilities outstanding with the same counterparty on the Condensed Consolidated Balance Sheets, for the contracts that contain netting provisions. See Note 7 – Derivative Contracts for additional information regarding the Company's derivative contracts.