Asset Retirement Obligation (Details) (USD $)	3 Months Ended		12 Months Ended	46 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Mar. 31, 2012
Asset retirement obligation activity
Balance at beginning of period	$ 15,541,000	$ 12,471,000	$ 12,471,000
Obligations settled	(124,000)		(1,030,000)
Accretion expense	251,000	234,000	955,000	3,374,000
Revisions in estimated cash flows	1,919,000		2,508,000
Gain on settlement			637,000
Balance at end of period	17,587,000		15,541,000	17,587,000
Financial assurance requirement satisfied with surety bonds	$ 28,800,000			$ 28,800,000