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Commitments and Contingencies, Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 27,266	$ 21,131
Liabilities settled	0	(514)
Accretion	1,675	1,428
Revisions to prior estimates	(11,334)	5,221
Ending balance	17,607	27,266
Electric Plant - Steam Production Asbestos [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	20,803	19,960
Liabilities settled	0	(514)
Accretion	1,422	1,357
Revisions to prior estimates	(11,246)	0
Ending balance	10,979	20,803
Electric Plant - Steam Production Ash Containment [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	719	345
Liabilities settled	0	0
Accretion	45	22
Revisions to prior estimates	0	352
Ending balance	764	719
Electric Plant - Electric Transmission and Distribution [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	5,744	826
Liabilities settled	0	0
Accretion	208	49
Revisions to prior estimates	(88)	4,869
Ending balance	$ 5,864	$ 5,744

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

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