Regulatory Matters (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Regulatory Assets
Deferred construction accounting costs	$ 16,167	$ 16,277
Iatan1
Regulatory Assets
Deferred construction accounting costs	5,427	5,469
Deferred construction accounting costs, current	(171)	(170)
Iatan1 | Deferred Carrying Charges
Regulatory Assets
Deferred construction accounting costs	2,657	2,678
Iatan1 | Deferred O&M
Regulatory Assets
Deferred construction accounting costs	1,330	1,339
Iatan1 | Depreciation
Regulatory Assets
Deferred construction accounting costs	1,611	1,622
Iatan 2
Regulatory Assets
Deferred construction accounting costs	10,381	10,437
Deferred construction accounting costs, current	(224)	(224)
Iatan 2 | Deferred Carrying Charges
Regulatory Assets
Deferred construction accounting costs	3,805	3,821
Iatan 2 | Deferred O&M
Regulatory Assets
Deferred construction accounting costs	4,126	4,155
Iatan 2 | Depreciation
Regulatory Assets
Deferred construction accounting costs	2,674	2,685
Plum Point
Regulatory Assets
Deferred construction accounting costs	359	371
Deferred construction accounting costs, current	(46)	(46)
Plum Point | Deferred Carrying Charges
Regulatory Assets
Deferred construction accounting costs	64	64
Plum Point | Deferred O&M
Regulatory Assets
Deferred construction accounting costs	183	195
Plum Point | Depreciation
Regulatory Assets
Deferred construction accounting costs	$ 158	$ 158