Asset Retirement Obligations (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Asset retirement obligation [Roll Forward]
Balance, beginning of period	$ 98,151,000	$ 95,970,000
Liabilities incurred	6,523,000	3,870,000
Liabilities acquired	0	0
Asset retirement obligation, liabilities settled	(10,472,000)	(10,418,000)
Accretion expense	4,266,000	4,466,000
Revisions in estimates	3,655,000	3,921,000
Other	422,000	342,000
Balance, end of period	102,545,000	98,151,000
Fair value of assets legally restricted for purposes of settling asset retirement obligations	$ 5,000,000	$ 5,700,000