Derivative Instruments and Hedging Activities - Before Tax Effect of Derivative Instruments in Statement of Operations (Detail) (Cash Flow Hedging [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	$ 47.1	$ (30.8)	$ 3.7	$ (20.9)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	(1.3)	(2.1)	(17.6)	13.8
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	2.1	(1.7)	0.5	(1.5)
Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	47.1	(30.8)	3.7	(20.9)
Derivative income (expense), net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	2.1	(1.7)	0.5	(1.5)
Operating revenue - oil/gas production [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	$ (1.3)	$ (2.1)	$ (17.6)	$ 13.8