Derivative Instruments and Hedging Activities - Gains or Losses Related to Changes in Fair Value and Cash Settlements on Derivatives Not Qualifying as Hedging Instruments (Details) - Not Designated as Hedging Instrument - Commodity contracts - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Cash settlements	$ 0.0	$ 3.8	$ 0.0	$ 11.0
Change in fair value	0.0	(2.6)	0.0	(7.9)
Total gains (losses) on non-qualifying hedges	$ 0.0	$ 1.2	$ 0.0	$ 3.1