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) (Commodity Contracts [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Cash settlements on Derivative Instruments not Qualifying Hedges	$ 3.1
Change in fair value on Derivative Instruments not Qualifying Hedges	(0.9)
Total gains on Derivative Instruments not Qualifying Hedges	$ 2.2