Regulatory Matters (Details 2) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Regulatory Assets
Deferred construction accounting costs	$ 16,811	$ 17,095
Iatan1
Regulatory Assets
Deferred construction accounting costs	5,665	5,743
Iatan1 | Deferred Carrying Charges
Regulatory Assets
Deferred construction accounting costs	2,690	2,728
Iatan1 | Deferred O&M
Regulatory Assets
Deferred construction accounting costs	1,345	1,363
Iatan1 | Depreciation.
Regulatory Assets
Deferred construction accounting costs	1,630	1,652
Iatan2
Regulatory Assets
Deferred construction accounting costs	10,718	10,890
Iatan2 | Deferred Carrying Charges
Regulatory Assets
Deferred construction accounting costs	3,839	3,891
Iatan2 | Deferred O&M
Regulatory Assets
Deferred construction accounting costs	4,184	4,271
Iatan2 | Depreciation.
Regulatory Assets
Deferred construction accounting costs	2,695	2,728
Plum point
Regulatory Assets
Deferred construction accounting costs	428	462
Plum point | Deferred Carrying Charges
Regulatory Assets
Deferred construction accounting costs	64	65
Plum point | Deferred O&M
Regulatory Assets
Deferred construction accounting costs	206	239
Plum point | Depreciation.
Regulatory Assets
Deferred construction accounting costs	$ 158	$ 158