Commitments and Contingencies (Details 2) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Commitments And Contingencies (Textual) [Abstract]
Total rental expense	$ 1,122,000	$ 773,000	$ 3,179,000	$ 2,506,000
Quantity of natural gas (cubic feet)	2,373,000,000		2,373,000,000
Wyoming Interstate Gas Company [Member]
Long Term Supply Commitment [Line Items]
Expiry date of agreements	Dec. 31, 2019		Dec. 31, 2019
Fort Union Gas Gathering [Member]
Long Term Supply Commitment [Line Items]
Expiry date of agreements	Nov. 30, 2017		Nov. 30, 2017
Formosa [Member]
Long Term Supply Commitment [Line Items]
Expiry date of agreements	Mar. 31, 2023		Mar. 31, 2023