Derivative Instruments and Hedging Activities - Pro Forma Impact Of Offsetting Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivative Liability [Abstract]
Gross liability	$ (243.8)	$ (99.3)
Pro forma net presentation, liability, total	(148.1)	(43.9)
Counterparties with Offsetting Positions or Collateral [Member]
Derivative Liability [Abstract]
Gross liability	(205.8)	(92.0)
Pro forma net presentation, liability, total	(110.1)	(36.6)
Counterparties without Offsetting Position [Member]
Derivative Liability [Abstract]
Gross liability	(38.0)	(7.3)
Pro forma net presentation, liability, total	(38.0)	(7.3)
Current Liabilities from Risk Management Activities [Member]
Derivative Liability [Abstract]
Gross liability	(176.3)	(79.7)
Pro forma net presentation, liability, current	(89.0)	(32.7)
Current Liabilities from Risk Management Activities [Member] \| Counterparties with Offsetting Positions or Collateral [Member]
Derivative Liability [Abstract]
Gross liability	(163.6)	(74.7)
Pro forma net presentation, liability, current	(76.3)	(27.7)
Current Liabilities from Risk Management Activities [Member] \| Counterparties without Offsetting Position [Member]
Derivative Liability [Abstract]
Gross liability	(12.7)	(5.0)
Pro forma net presentation, liability, current	(12.7)	(5.0)
Long-Term Liabilities from Risk Management Activities [Member]
Derivative Liability [Abstract]
Gross liability	(67.5)	(19.6)
Pro forma net presentation, liability, noncurrent	(59.1)	(11.2)
Long-Term Liabilities from Risk Management Activities [Member] \| Counterparties with Offsetting Positions or Collateral [Member]
Derivative Liability [Abstract]
Gross liability	(42.2)	(17.3)
Pro forma net presentation, liability, noncurrent	(33.8)	(8.9)
Long-Term Liabilities from Risk Management Activities [Member] \| Counterparties without Offsetting Position [Member]
Derivative Liability [Abstract]
Gross liability	(25.3)	(2.3)
Pro forma net presentation, liability, noncurrent	$ (25.3)	$ (2.3)

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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-Publisher FASB
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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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-Publisher FASB
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