Property, Plant and Equipment (details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant and Equipment, Gross	$ 2,755,729	$ 2,661,102	$ 2,632,498
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	798,510	767,226	756,592
Total property, plant and equipment - net	1,957,219	1,893,876	1,875,906
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant and Equipment, Gross	2,399,600	2,323,467	2,296,788
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	53,017	36,051	36,459
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	776,812	749,603	740,378
Total property, plant and equipment - net	1,675,805	1,609,915	1,592,869
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant and Equipment, Gross	296,486	293,205	290,075
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	6,626	8,379	9,176
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	21,698	17,623	16,214
Total property, plant and equipment - net	$ 281,414	$ 283,961	$ 283,037

The cumulative amount of depreciation, depletion and amortization (related to property, plant and equipment, but not including land) that has been recognized in the income statement.

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Gross amount, at the balance sheet date, of long-lived physical assets used in the normal conduct of business and not intended for resale. This can include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software.

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- Definition

Tangible assets that are held by an entity for use in the production or supply of goods and services, for rental to others, or for administrative purposes and that are expected to provide economic benefit for more than one year; net of accumulated depreciation. Examples include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software.

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