Derivative Instruments and Hedging Activities - Pro Forma Impact Of Offsetting Assets (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative Asset [Abstract]
Gross asset	$ 104.6	$ 61.1
Derivative assets	104.6	61.1
Pro forma net presentation, asset, total	72.3	28.6
Counterparties with Offsetting Positions or Collateral [Member]
Derivative Asset [Abstract]
Gross asset	104.6	61.1
Pro forma net presentation, asset	72.3	28.6
Current Assets from Risk Management Activities [Member]
Derivative Asset [Abstract]
Gross asset	63.2	37.9
Pro forma net presentation, asset, current	39.6	13.8
Current Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]
Derivative Asset [Abstract]
Gross asset	63.2	37.9
Pro forma net presentation, asset	39.6	13.8
Long-Term Assets from Risk Management Activities [Member]
Derivative Asset [Abstract]
Gross asset	41.4	23.2
Pro forma net presentation, asset, noncurrent	32.7	14.8
Long-Term Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member]
Derivative Asset [Abstract]
Gross asset	41.4	23.2
Pro forma net presentation, asset	$ 32.7	$ 14.8