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 [Member] - Commodity Contracts [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Cash settlements on Derivative Instruments not Qualifying Hedges	$ 3.8	$ 0.7	$ 11.0	$ 0.7
Change in fair value on Derivative Instruments not Qualifying Hedges	(2.6)	3.0	(7.9)	1.5
Total gains (losses) on Derivative Instruments not Qualifying Hedges	$ 1.2	$ 3.7	$ 3.1	$ 2.2