Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Regulated Asset Liability [Line Items]
Earning a return, asset	$ 448,712
Not earning a return, asset	219,990
Pending regulatory treatment, asset	95,414
Regulatory assets	13,673	$ 21,851
Non-current regulatory assets	750,443	670,802
Earning a return, liability	785,088
Not earning a return, liability	35,218
Pending Regulatory Treatment Liability	10,949
Regulatory liabilities	46,435	51,715
Non-current regulatory liabilities	784,820	775,436
Natural Gas Deferrals [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	874
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	874	3,189
Non-current regulatory liabilities	0	0
Power Deferrals Regulatory Liability [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	37,869
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	20,299	14,155
Non-current regulatory liabilities	17,570	23,544
Removal Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	325,832
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	0	0
Non-current regulatory liabilities	325,832	312,403
Deferred Income Tax Charge [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	390,056
Not earning a return, liability	24,573
Pending Regulatory Treatment Liability	0
Regulatory liabilities	14,952	23,803
Non-current regulatory liabilities	399,677	407,549
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	15,046
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	0	0
Non-current regulatory liabilities	$ 15,046	17,088
Decoupling Surcharge [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory liability	2 years
Earning a return, liability	$ 2,966
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Regulatory liabilities	1,447	255
Non-current regulatory liabilities	1,519	2,398
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	12,445
Not earning a return, liability	10,645
Pending Regulatory Treatment Liability	0
Regulatory liabilities	8,863	10,313
Non-current regulatory liabilities	14,227	12,454
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	0
Not earning a return, liability	0
Pending Regulatory Treatment Liability	10,949
Regulatory liabilities	0	0
Non-current regulatory liabilities	10,949	0
Deferred Income Tax Charge [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	108,517
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	108,517	95,752
Pension and Other Postretirement Benefit Plans [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	198,746
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	198,746	208,754
Energy commodity derivatives
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	7,795
Pending regulatory treatment, asset	0
Regulatory assets	2,073	6,310
Non-current regulatory assets	5,722	264
Unamortized Debt Repurchase Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	7,512
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	$ 7,512	8,884
Settlement with Coeur d'Alene Tribe [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	39 years
Earning a return, asset	$ 40,043
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	40,043	41,332
Demand Side Management Programs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	3,814
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	$ 3,814	12,170
Decoupling Surcharge [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	2 years
Earning a return, asset	$ 24,246
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	7,123	12,098
Non-current regulatory assets	17,123	14,806
Utility Plant to be Abandoned [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	28,916
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	28,916	31,291
Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	130,538
Not earning a return, asset	0
Pending regulatory treatment, asset	84,313
Regulatory assets	0	0
Non-current regulatory assets	214,851	168,594
Deferred power costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	3,337
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	1,775	0
Non-current regulatory assets	1,562	0
AFUDC Above FERC Allowed Rate [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	47,393
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	47,393	40,749
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	31,831
Not earning a return, asset	9,635
Pending regulatory treatment, asset	2,935
Regulatory assets	2,702	3,443
Non-current regulatory assets	41,699	34,811
COVID-19 Deferrals [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	0
Pending regulatory treatment, asset	8,166
Regulatory assets	0	0
Non-current regulatory assets	8,166	0
Advanced Meter Infrastructure [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	26,379
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Regulatory assets	0	0
Non-current regulatory assets	$ 26,379	$ 13,395

Pending regulatory treatment, liability

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Regulated Asset Liability [Line Items]

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Earning a return includes either interest on the regulatory asset, or a return on the investment as a component of rate base or the weighted cost of capital.

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Earning a return includes either interest on the regulatory liability, or a return on the investment as a component of rate base or the weighted cost of capital.

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Remaining amounts of regulatory liabilities for which no interest during recovery period is provided.

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Amount of regulatory costs not yet approved by the regulatory authority.

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Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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Amortization period for the recovery of regulatory liability, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

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The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

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Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base.

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