Balance Sheet Components Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Regulated Asset Liability [Line Items]
Regulatory assets	$ 19,890	$ 48,552
Regulatory Assets, Noncurrent	688,814	614,354
Regulatory liabilities	43,442	113,209
Non-current regulatory liabilities	786,131	780,701
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory liabilities	23,409	27,997
Non-current regulatory liabilities	414,857	425,613
Natural Gas Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	3,762	40,713
Non-current regulatory liabilities	0	0
Power Deferrals Regulatory Liability [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	7,711	25,072
Non-current regulatory liabilities	29,618	16,933
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory liabilities	99	6,782
Non-current regulatory liabilities	3,098	204
Removal Costs [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	307,923	297,379
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory liabilities	0	0
Non-current regulatory liabilities	16,863	28,078
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory liabilities	8,461	12,645
Non-current regulatory liabilities	13,772	12,494
Energy commodity derivatives
Regulated Asset Liability [Line Items]
Regulatory assets	6,374	41,428
Regulatory Assets, Noncurrent	1,018	16,866
Decoupling surcharge
Regulated Asset Liability [Line Items]
Regulatory assets	10,266	3,408
Regulatory Assets, Noncurrent	18,395	17,501
Pension and other postretirement benefit plans
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	218,006	228,062
Interest rate swaps
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	189,872	133,854
Income Tax Related
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	93,818	91,188
Settlement with Coeur d'Alene Tribe
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	41,660	42,643
AFUDC above FERC allowed rate (1)
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	37,239	1,814
Demand side management programs
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	11,701	19,674
Utility plant to be abandoned
Regulated Asset Liability [Line Items]
Regulatory assets	0	0
Regulatory Assets, Noncurrent	25,849	24,334
Other Regulatory Assets (Liabilities) [Member]
Regulated Asset Liability [Line Items]
Regulatory assets	3,250	3,716
Regulatory Assets, Noncurrent	$ 51,256	$ 38,418