Environmental Matters (Details)		3 Months Ended
		Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)
Environmental Activity [Line Items]
Accrual for Environmental Loss Contingencies, Discount Rate		1.00%
Accrual for Environmental Loss Contingencies, Annual Amortization Rate		0.20
Site Contingency, Recovery from Third Party of Environmental Cost, Interest Rate on Recognition		1.00%
Remediation recovery percentage in Oregon		96.68%
Portland Harbor EPA Clean-Up Present Value Cost		$ 1,050,000,000
Portland Harbor Number Of Potentially Responsible Parties		more than 100
Loss Contingency Range Of Possible Loss Gasco Siltronic		$44.8 million to $350 million
Gasco Siltronic Sediments Project Liability		$ 44,800,000
NRD Liability Claim Yakama Nation		300,000
Front Street liability for studies and design costs		1,000,000
SRRM pre-tax regulatory disallowance		15,000,000
Deferred environmental charges		95,000,000
Total SRRM disallowance from February 2016 Order		3,300,000
Annual tariff rider collection		5,000,000
Environmental Settlement Insurance Proceeds Received To Date		150,000,000
Annual insurance proceeds to apply against remediation costs		$ 5,000,000
Site Contingency, Recovery from Third Party of Environmental Cost, Recognition Term		20 years
Insurance proceeds applied to prudently incurred remediation costs		$ 63,200,000
Remediation non-recovery percentage for Oregon		3.32%
Annual Insurance Proceeds to Apply Against Remediation Costs		$ 5,000,000
Total Annual Remediation Expense and Interest		10,000,000
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		14,638,000	$ 14,475,000	$ 16,124,000
Environmental Noncurrent Liabilities		105,285,000	105,268,000	106,182,000
Regulatory Assets, Current		34,874,000	42,362,000	61,524,000
Regulatory Assets, Noncurrent		349,057,000	357,530,000	351,390,000
Minimum [Member]
Environmental Activity [Line Items]
Portland Harbor EPA Clean-Up Present Value Cost		(0.30)
Front Street Remedial Costs		7,600,000
Maximum [Member]
Environmental Activity [Line Items]
Portland Harbor EPA Clean-Up Present Value Cost		0.50
Front Street Remedial Costs		12,900,000
Gasco Siltronic Sediments [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		1,573,000	869,000	2,747,000
Environmental Noncurrent Liabilities		43,200,000	43,972,000	42,079,000
Portland Harbor Other [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		1,804,000	1,970,000	1,655,000
Environmental Noncurrent Liabilities		3,940,000	4,148,000	4,775,000
Gasco Upland [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		10,335,000	10,657,000	10,626,000
Environmental Noncurrent Liabilities		50,189,000	49,183,000	51,403,000
Central Service Center [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		68,000	73,000	25,000
Environmental Noncurrent Liabilities		0	0	0
Front Street [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		858,000	906,000	1,071,000
Environmental Noncurrent Liabilities		7,777,000	7,786,000	7,746,000
Oregon Steel Mills [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		0	0	0
Environmental Noncurrent Liabilities		179,000	179,000	179,000
Environmental Restoration Costs [Member]
Environmental Activity [Line Items]
Environmental Regulatory Deferred Cost and Interest	[1]	49,373,000	53,039,000	57,359,000
Environmental Regulatory Insurance Proceeds and Interest		(99,195,000)	(98,523,000)	(102,570,000)
Environmental Regulatory Accrued Site Liabilities	[2]	119,623,000	119,443,000	122,306,000
Environmental Regulatory Table [Abstract]
Environmental Regulatory Assets Noncurrent	[1]	69,801,000	73,959,000	77,095,000
Accrual for Environmental Loss Contingencies [Abstract]
Regulatory Assets, Current	[3],[4]	7,574,000	9,989,000	9,096,000
Regulatory Assets, Noncurrent	[3],[4]	62,227,000	$ 63,970,000	$ 67,999,000
Environmental Restoration Costs [Member] | Front Street [Member]
Environmental Activity [Line Items]
Environmental Regulatory Accrued Site Liabilities		$ 300,000

[1] Includes pre-review and post-review deferred costs, amounts currently in amortization, and interest, net of amounts collected from customers.

[2] Excludes $0.3 million, or 3.32% of the Front Street site liability as the OPUC allows recovery of 96.68% of costs for those sites allocable to Oregon, including those that historically served only Oregon customers.

[3] Environmental costs relate to specific sites approved for regulatory deferral by the OPUC and WUTC. In Oregon, we earn a carrying charge on cash amounts paid, whereas amounts accrued but not yet paid do not earn a carrying charge until expended. We also accrue a carrying charge on insurance proceeds for amounts owed to customers. In Washington, a carrying charge related to deferred amounts will be determined in a future proceeding. Current environmental costs represent remediation costs management expects to collect from customers in the next 12 months. Amounts included in this estimate are still subject to a prudence and earnings test review by the OPUC and do not include the $5 million tariff rider. The amounts allocable to Oregon are recoverable through utility rates, subject to an earnings test.

[4] Environmental costs relate to specific sites approved for regulatory deferral by the OPUC and WUTC. In Oregon, we earn a carrying charge on cash amounts paid, whereas amounts accrued but not yet paid do not earn a carrying charge until expended. We also accrue a carrying charge on insurance proceeds for amounts owed to customers. In Washington, recovery of deferred amounts will be determined in a future proceeding. Current environmental costs represent remediation costs management expects to collect from Oregon customers in the next 12 months. Amounts included in this estimate are still subject to a prudence and earnings test review by the OPUC and do not include the $5 million tariff rider. The amounts allocable to Oregon are recoverable through utility rates, subject to the aforementioned earnings test. See Note 13.