Effect of Derivative Instruments in Statement of Operations (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	$ (7.4)	$ (12.8)
Derivative Instruments, Gain (Loss) Reclassified in Other Comprehensive Income (Loss), Effective Portion, Net, Total	(3.1)	0.4
Oil
Derivative Instruments, Gain (Loss) [Line Items]
Decrease/increase in oil revenue owing to effective hedging contracts	4.5	5.8
Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Decrease/increase in gas revenue owing to effective hedging contracts	$ 4.0	$ 4.8