Derivative Instruments and Hedging Activities - Before Tax Effect of Derivative Instruments in Statement of Income (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Decrease/increase in oil revenue owing to effective hedging contracts	$ 6.1	$ 5.3	$ 8.6	$ 9.9
Decrease/increase in gas revenue owing to effective hedging contracts	$ 3.1	$ 2.0	$ 7.7	$ 6.0