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, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	$ (30.8)		$ (41.8)		$ (20.9)		$ 30.8
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	(2.1)	[1]	10.0	[1]	13.8	[2]	18.4	[2]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	(1.7)		(1.8)		(1.5)		3.1
Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	(30.8)		(41.8)		(20.9)		30.8
Derivative (expense) income, 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	(1.7)		(1.8)		(1.5)		3.1
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	$ (2.1)	[1]	$ 10.0	[1]	$ 13.8	[2]	$ 18.4	[2]

[1] For the three months ended September 30, 2013, effective hedging contracts decreased oil revenue by $7.5 million and increased gas revenue by $5.4 million. For the three months ended September 30, 2012, effective hedging contracts increased oil revenue by $4.1 million and increased gas revenue by $5.9 million.

[2] For the nine months ended September 30, 2013, effective hedging contracts increased oil revenue by $2.4 million and increased gas revenue by $11.4 million. For the nine months ended September 30, 2012, effective hedging contracts increased oil revenue by $1.1 million and increased gas revenue by $17.3 million.