Balance Sheet Components - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 06, 2020	Dec. 31, 2019
Regulated Asset Liability [Line Items]
Regulatory assets	$ 11,177		$ 21,851
Non-current regulatory assets	752,090		670,802
Regulatory liabilities	49,629		51,715
Non-current regulatory liabilities	778,826		775,436
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory liabilities	13,335		23,803
Non-current regulatory liabilities	400,564		407,549
Natural Gas Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	2,778		3,189
Non-current regulatory liabilities	0		0
Power Deferrals Regulatory Liability [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	21,692		14,155
Non-current regulatory liabilities	20,112		23,544
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory liabilities	796		255
Non-current regulatory liabilities	3,359		2,398
Provision for Rate Refunds [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	7,339	$ 8,500	3,565
Non-current regulatory liabilities	0		0
Removal Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	0		0
Non-current regulatory liabilities	322,138		312,403
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory liabilities	0		0
Non-current regulatory liabilities	15,773		17,088
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	3,689		6,748
Non-current regulatory liabilities	16,880		12,454
Energy commodity derivatives
Regulated Asset Liability [Line Items]
Regulatory assets	309		6,310
Non-current regulatory assets	0		264
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory assets	10,345		12,098
Non-current regulatory assets	18,514		14,806
Pension and other postretirement benefit plans
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	203,471		208,754
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	244,672		168,594
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	98,879		95,752
Settlement with Coeur d'Alene Tribe
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	40,677		41,332
AFUDC above FERC allowed rate
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	41,912		40,749
Demand side management programs
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	6,538		12,170
Utility plant to be abandoned
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	30,700		31,291
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	523		3,443
Non-current regulatory assets	$ 66,727		$ 57,090