Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Land, land rights and easements	$ 149,636	$ 148,507
Buildings and improvements	264,205	242,038
Transmission, distribution and storage	3,341,001	3,163,463
Generation	1,193,117	1,187,346
Plant acquisition adjustment	686,328	685,417
Other	541,741	521,711
Construction work in progress	110,076	69,902
Total property, plant and equipment	6,286,104	6,018,384
Less accumulated depreciation	(1,764,786)	(1,660,119)
Net property, plant, and equipment	4,521,318	4,358,265
Property, plant, and equipment under capital leases	$ 15,400	17,500
Land and improvements [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	50 years
Land and improvements [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	96 years
Building and improvements [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	26 years
Building and improvements [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	64 years
Tranmission, distribution and storage[Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	15 years
Tranmission, distribution and storage[Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	85 years
Generation [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	25 years
Generation [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	50 years
Plant Acquisition adjustment [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	25 years
Plant Acquisition adjustment [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	50 years
Other [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	2 years
Other [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	45 years
Basin Capital Lease [Member]
Property, Plant and Equipment [Line Items]
Property, plant, and equipment under capital leases	$ 15,100	$ 17,100

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Amount before accumulated depreciation of building structures held for productive use including addition, improvement, or renovation to the structure, including, but not limited to, interior masonry, interior flooring, electrical, and plumbing.

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Amount before depreciation of leased long-lived, physical assets used to produce goods and services and not intended for resale meeting the criteria for capitalization, classified as other.

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Amount before accumulated deprecation and depletion of real estate held for productive use and additions or improvements to real estate held for productive use, examples include, but are not limited to, walkways, driveways, fences, and parking lots. Excludes land held for sale.

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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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Amount of acquisition adjustments included in property, plant and equipment (PPE) for utilities.

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Period end amount of construction work in progress in public utility.

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Period end amount of property, plant and equipment (PPE) related to generation or processing owned by public utility.

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Period end book value of other property, plant and equipment (PPE) owned (but not classified elsewhere) by the public utility.

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Period end amount of property, plant and equipment (PPE) related to transmission and distribution owned by public utility.

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