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	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2014
Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Cash settlements on Derivative Instruments not Qualifying Hedges	$ 0.7	$ 0.7
Change in fair value on Derivative Instruments not Qualifying Hedges	3.0	1.5
Total gains on Derivative Instruments not Qualifying Hedges	$ 3.7	$ 2.2