Regulatory Matters (Details 3) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Regulatory Assets
Deferred construction accounting costs	$ 16,496	$ 16,717
Iatan1
Regulatory Assets
Deferred construction accounting costs	5,554	5,639
Iatan1 | Deferred Carrying Charges
Regulatory Assets
Deferred construction accounting costs	2,637	2,678
Iatan1 | Deferred O&M
Regulatory Assets
Deferred construction accounting costs	1,319	1,339
Iatan1 | Depreciation
Regulatory Assets
Deferred construction accounting costs	1,598	1,622
Iatan 2
Regulatory Assets
Deferred construction accounting costs	10,548	10,661
Iatan 2 | Deferred Carrying Charges
Regulatory Assets
Deferred construction accounting costs	3,788	3,821
Iatan 2 | Deferred O&M
Regulatory Assets
Deferred construction accounting costs	4,097	4,155
Iatan 2 | Depreciation
Regulatory Assets
Deferred construction accounting costs	2,663	2,685
Plum Point
Regulatory Assets
Deferred construction accounting costs	394	417
Plum Point | Deferred Carrying Charges
Regulatory Assets
Deferred construction accounting costs	64	64
Plum Point | Deferred O&M
Regulatory Assets
Deferred construction accounting costs	173	195
Plum Point | Depreciation
Regulatory Assets
Deferred construction accounting costs	$ 157	$ 158