Property, Plant and Equipment (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Land, land rights and easements		$ 156,069	$ 149,636
Buildings and improvements		278,164	264,205
Transmission, distribution and storage		3,569,141	3,341,001
Generation		1,222,796	1,193,117
Plant acquisition adjustment	[1]	686,328	686,328
Other		545,009	541,741
Construction work in progress		96,421	110,076
Total property, plant and equipment		6,553,928	6,286,104
Less accumulated depreciation		(1,853,004)	(1,764,786)
Net property, plant, and equipment		4,700,924	4,521,318
Property, plant, and equipment under capital leases		$ 13,300	15,400
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		23 years
Building and improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life		73 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		23 years
Generation [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life		71 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		$ 13,100	$ 15,100

[1] The plant acquisition adjustment balance above includes our Beethoven wind project acquired in 2015, our hydro generating assets acquired in 2014, and the inclusion of our interest in Colstrip Unit 4 in rate base in 2009. The acquisition adjustment is amortized on a straight-line basis over the estimated remaining useful life of each related asset in depreciation expense.