Effect of Derivative Instruments in Statement of Operations (Detail) (Cash Flow Hedging, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	$ (12.9)	[1]	$ (22.7)	[1]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	5.4	[2]	(0.6)	[2]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	(1.2)		(0.5)
Commodity Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	(12.9)	[1]	(22.7)	[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	(1.2)		(0.5)
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	$ 5.4	[2]	$ (0.6)	[2]

[1] Net of related tax effect of ($7.4) million and ($12.8) million for the three months ended March 31, 2013 and 2012, respectively.

[2] Net of related tax effect of ($3.1) million and $0.4 million for the three months ended March 31, 2013 and 2012, respectively. For the three months ended March 31, 2013, effective hedging contracts increased oil revenue by $4.5 million and increased gas revenue by $4.0 million. For the three months ended March 31, 2012, effective hedging contracts decreased oil revenue by $5.8 million and increased gas revenue by $4.8 million.