Asset Retirement Obligations (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligations [Roll Forward]
Asset retirement obligation, beginning balance	$ 19,913,000	$ 16,513,000
Liabilities incurred	85,000	439,000
Accretion expense	1,071,000	1,093,000
Liabilities settled	(157,000)	(98,000)
Revisions in estimated cash flows	(1,739,000)	1,966,000
Asset retirement obligation, ending balance	19,173,000	19,913,000
Revisions in estimated cash flows [Abstract]
Revision of estimate for Columbia Landfill ARO	(1,600,000)	700,000
Revision of estimate for Wind Generation Facilities ARO		1,200,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities	20,463,000	23,772,000
Non-ARO removal cost [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 13,116,000	$ 12,285,000