Effect of Derivative Instruments in Statement of Operations (Detail) (Cash Flow Hedging, USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	$ 30.0		$ 108.1		$ 9.9		$ 72.6
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	7.3	[1]	9.4	[1]	15.9	[2]	8.4	[2]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	1.4		5.4		0.1		4.9
Commodity Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	30.0		108.1		9.9		72.6
Derivative income, net
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	1.4		5.4		0.1		4.9
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	$ 7.3	[1]	$ 9.4	[1]	$ 15.9	[2]	$ 8.4	[2]

[1] For the three months ended June 30, 2013, effective hedging contracts increased oil revenue by $5.3 million and increased gas revenue by $2.0 million. For the three months ended June 30, 2012, effective hedging contracts increased oil revenue by $2.9 million and increased gas revenue by $6.5 million.

[2] For the six months ended June 30, 2013, effective hedging contracts increased oil revenue by $9.9 million and increased gas revenue by $6.0 million. For the six months ended June 30, 2012, effective hedging contracts decreased oil revenue by $2.9 million and increased gas revenue by $11.3 million.