Fair Value Measurements Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2017	Dec. 31, 2016
Financial Assets
Derivative Asset			$ 5.5	$ 0.0
Net amount of derivative assets presented on condensed consolidated balance sheets	[1]		(3.9)	0.0
Derivative Liability			15.8	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			9.4	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross			19.7	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
Long-term [Member]
Financial Assets
Derivative Asset			1.7	0.0
Net amount of derivative assets presented on condensed consolidated balance sheets	[1]		(0.8)	0.0
Derivative Liability			2.4	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			2.5	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross			3.2	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
Short-term [Member]
Financial Assets
Derivative Asset			3.8	0.0
Net amount of derivative assets presented on condensed consolidated balance sheets	[1]		(3.1)	0.0
Derivative Liability			13.4	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			6.9	0.0
Financial liabilities
Derivative Liability, Fair Value, Gross			16.5	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
Proved Property [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Impairment		$ 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.