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	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Cash settlements on Derivative Instruments not Qualifying Hedges	$ 4.1		$ 7.2
Change in fair value on Derivative Instruments not Qualifying Hedges	(4.4)	$ (1.5)	(5.3)	$ (1.5)
Total gains (losses) on Derivative Instruments not Qualifying Hedges	$ (0.3)	$ (1.5)	$ 1.9	$ (1.5)