Derivative Instruments and Hedging Activities - Before Tax Effect of Derivative Instruments in Statement of Operations (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Decrease/increase in oil revenue owing to effective hedging contracts	$ 34.0	$ (2.5)
Decrease/increase in gas revenue owing to effective hedging contracts	$ 2.8	$ (4.6)