Derivative Instruments and Hedging Activities (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Effect of Derivative Instruments on the Statement of Operations
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total		$ 20.8	$ 23.7	$ 2.1
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	38.1
Cash Flow Hedging [Member]
Effect of Derivative Instruments on the Statement of Operations
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	63.2	36.1	41.1	3.6
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	9.4	(10.5)	8.4	(14.4)
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	5.4	1.4	4.9	(0.8)
Cash Flow Hedging [Member] | Commodity contracts [Member]
Effect of Derivative Instruments on the Statement of Operations
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	63.2	36.1	41.1	3.6
Operating revenue - oil/gas production [Member] | Cash Flow Hedging [Member]
Effect of Derivative Instruments on the Statement of Operations
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	9.4	(10.5)	8.4	(14.4)
Derivative income, net [Member] | Cash Flow Hedging [Member]
Effect of Derivative Instruments on the Statement of Operations
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	$ 5.4	$ 1.4	$ 4.9	$ (0.8)