Asset Retirement Obligations (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Asset retirement obligation [Roll Forward]
Balance, beginning of period	$ 95,970,000	$ 76,359,000
Liabilities incurred	3,870,000	8,608,000
Liabilities acquired	0	5,272,000
Asset Retirement Obligation, Liabilities Settled	(10,418,000)	(10,740,000)
Accretion expense	4,466,000	3,588,000
Revisions in estimates	3,921,000	12,621,000
Other	342,000	262,000
Balance, end of period	98,151,000	95,970,000
Fair value of assets legally restricted for purposes of settling asset retirement obligations	$ 5,700,000	$ 5,700,000