Derivative Instruments and Hedging Activities - Before Tax Effect of Derivative Instruments in Statement of Operations - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Decrease/increase in oil revenue owing to effective hedging contracts	$ 36.3	$ (1.3)	$ 96.8	$ (10.0)
Decrease/increase in gas revenue owing to effective hedging contracts	$ (3.6)		$ 10.3	$ (7.6)