Effect of Derivative Instruments in Statement of Operations (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	$ (1.8)		$ 4.1
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	(33.2)	[1]	76.0	[1]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	10.0	[2]	1.3	[2]	18.4		(13.1)
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	(33.2)	[1]	76.0	[1]
Operating revenue - oil/gas production
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	10.0	[2]	1.3	[2]	18.4		(13.1)
Derivative income (expense), net
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	(1.8)		4.1
Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total					3.1		3.3
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total					7.9	[1]	79.6	[1]
Cash Flow Hedging | Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total					7.9	[1]	79.6	[1]
Cash Flow Hedging | Derivative income (expense), net
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total					$ 3.1		$ 3.3

[1] Net of related tax effect of ($19.2) million and $40.4 million for the three months ended September 30, 2012 and 2011, respectively.

[2] 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. For the three months ended September 30, 2011, effective hedging contracts decreased oil revenue by $3.3 million and increased gas revenue by $4.6 million.