Derivative Instruments (Schedule of Fair Value Holding Percentage) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2012	Dec. 31, 2011
Derivative Instruments, Fair Value [Line Items]
Derivative Asset, Fair Value, Net	$ 58.6	$ 37.3
Derivative Instrument Holding Percentage	100.00%	100.00%
Credit Suisse [Member]
Derivative Instruments, Fair Value [Line Items]
Derivative Instrument Holding Percentage	52.00%	68.00%
Bnp Paribas [Member]
Derivative Instruments, Fair Value [Line Items]
Derivative Instrument Holding Percentage	27.00%	19.00%
Societe Generale [Member]
Derivative Instruments, Fair Value [Line Items]
Derivative Instrument Holding Percentage	11.00%	2.00%
Bbva Compass [Member]
Derivative Instruments, Fair Value [Line Items]
Derivative Instrument Holding Percentage	6.00%	0.00%
Shell Energy North America Us Lp [Member]
Derivative Instruments, Fair Value [Line Items]
Derivative Instrument Holding Percentage	2.00%	6.00%
Credit Agricole [Member]
Derivative Instruments, Fair Value [Line Items]
Derivative Instrument Holding Percentage	2.00%	5.00%