Derivative Instruments and Hedging Activities - Pro Forma Impact Of Offsetting Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative Liability [Abstract]
Gross liability	$ (71.7)	$ (99.3)
Pro forma net presentation, liability, total	(33.6)	(43.9)
Counterparties with Offsetting Positions or Collateral [Member]
Derivative Liability [Abstract]
Gross liability	(68.1)	(92.0)
Pro forma net presentation, liability, total	(30.0)	(36.6)
Counterparties without Offsetting Position [Member]
Derivative Liability [Abstract]
Gross liability	(3.6)	(7.3)
Pro forma net presentation, liability, total	(3.6)	(7.3)
Current Liabilities from Risk Management Activities [Member]
Derivative Liability [Abstract]
Gross liability	(53.6)	(79.7)
Pro forma net presentation, liability, current	(24.2)	(32.7)
Current Liabilities from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]
Derivative Liability [Abstract]
Gross liability	(51.2)	(74.7)
Pro forma net presentation, liability, current	(21.8)	(27.7)
Current Liabilities from Risk Management Activities [Member] | Counterparties without Offsetting Position [Member]
Derivative Liability [Abstract]
Gross liability	(2.4)	(5.0)
Pro forma net presentation, liability, current	(2.4)	(5.0)
Long-Term Liabilities from Risk Management Activities [Member]
Derivative Liability [Abstract]
Gross liability	(18.1)	(19.6)
Pro forma net presentation, liability, noncurrent	(9.4)	(11.2)
Long-Term Liabilities from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]
Derivative Liability [Abstract]
Gross liability	(16.9)	(17.3)
Pro forma net presentation, liability, noncurrent	(8.2)	(8.9)
Long-Term Liabilities from Risk Management Activities [Member] | Counterparties without Offsetting Position [Member]
Derivative Liability [Abstract]
Gross liability	(1.2)	(2.3)
Pro forma net presentation, liability, noncurrent	$ (1.2)	$ (2.3)