Derivative Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 MMBTU	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Net asset, fair value	$ 29.8	$ 37.3
Positions not covered by put spread	8,185,600
Credit Suisse [Member]
Derivatives, Fair Value [Line Items]
Percentage of derivate asset owned	68.00%
BNP Paribas [Member]
Derivatives, Fair Value [Line Items]
Percentage of derivate asset owned	13.00%
Shell Energy North America (US) LP [Member]
Derivatives, Fair Value [Line Items]
Percentage of derivate asset owned	4.00%
Credit Agricole [Member]
Derivatives, Fair Value [Line Items]
Percentage of derivate asset owned	3.00%
Societe Generale [Member]
Derivatives, Fair Value [Line Items]
Percentage of derivate asset owned	10.00%
BBVA Compass [Member]
Derivatives, Fair Value [Line Items]
Percentage of derivate asset owned	2.00%