Effect of Derivative Instruments in Statement of Operations (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Decrease/increase in oil revenue owing to effective hedging contracts	$ 7.5	$ 4.1	$ 2.4	$ 1.1
Decrease/increase in gas revenue owing to effective hedging contracts	$ 5.4	$ 5.9	$ 11.4	$ 17.3