Derivative Instruments and Hedging Activities - Before Tax Effect of Derivative Instruments in Statement of Operations (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Decrease/increase in oil revenue owing to effective hedging contracts	$ 26.4	$ (6.1)	$ 60.4	$ (8.6)
Decrease/increase in gas revenue owing to effective hedging contracts	$ 4.0	$ (3.1)	$ 6.8	$ (7.7)