Derivative Instruments and Hedging Activities - Gains or Losses Related to Changes in Fair Value and Cash Settlements on Derivatives Not Qualifying as Hedging Instruments (Detail) - Not Designated as Hedging Instrument - Commodity contracts - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Cash settlements	$ 0.0	$ 3.1
Change in fair value	0.0	(0.9)
Total gain on non-qualifying hedges	$ 0.0	$ 2.2