Balance Sheet Components - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Regulated Asset Liability [Line Items]
Regulatory assets	$ 11,194		$ 21,851
Non-current regulatory assets	737,541		670,802
Regulatory liabilities	48,902		51,715
Non-current regulatory liabilities	773,641		775,436
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory liabilities	13,598		23,803
Non-current regulatory liabilities	397,082		407,549
Natural Gas Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	0		3,189
Non-current regulatory liabilities	0		0
Power Deferrals Regulatory Liability [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	22,384		14,155
Non-current regulatory liabilities	17,166		23,544
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory liabilities	1,663		255
Non-current regulatory liabilities	2,583		2,398
Provision for Rate Refunds [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	5,967	$ 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	322,129		312,403
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory liabilities	0		0
Non-current regulatory liabilities	15,409		17,088
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	5,290		6,748
Non-current regulatory liabilities	19,272		12,454
Energy commodity derivatives
Regulated Asset Liability [Line Items]
Regulatory assets	0		6,310
Non-current regulatory assets	0		264
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory assets	8,414		12,098
Non-current regulatory assets	16,742		14,806
Pension and other postretirement benefit plans
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	200,829		208,754
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	232,088		168,594
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	99,289		95,752
Settlement with Coeur d'Alene Tribe
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	40,349		41,332
AFUDC above FERC allowed rate
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	44,504		40,749
Demand side management programs
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	4,147		12,170
Utility plant to be abandoned
Regulated Asset Liability [Line Items]
Regulatory assets	0		0
Non-current regulatory assets	27,200		31,291
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	2,780		3,443
Non-current regulatory assets	$ 72,393		$ 57,090