Effect of Derivative Instruments in Statement of Operations (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Oil
Derivative Instruments, Gain (Loss) [Line Items]
Decrease/increase in oil revenue owing to effective hedging contracts	$ 5.3	$ 2.9	$ 9.9	$ 2.9
Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Decrease/increase in gas revenue owing to effective hedging contracts	$ 2.0	$ 6.5	$ 6.0	$ 11.3