Derivative Instruments and Hedging Activities - Location and Fair Value Amounts of Derivative Instruments Reported in Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Current liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	$ (15.3)
Long-term liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(0.4)
Current Asset [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	0.7
Long-term assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	1.9
Commodity Contracts [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	2.6	5.9
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	15.7	8.3
Commodity Contracts [Member] | Current liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	15.3	7.8
Commodity Contracts [Member] | Long-term liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	0.4	0.5
Commodity Contracts [Member] | Current Asset [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	0.7	4.5
Commodity Contracts [Member] | Long-term assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 1.9	$ 1.4