Environmental Matters (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Environmental Activity [Line Items]
Remediation non-recovery percentage for Oregon		3.32%
Remediation recovery percentage in Oregon		96.68%
Loss Contingency Range Of Possible Loss Portland Harbor		$791 million to $2.45 billion
Portland Harbor EPA Clean-Up Present Value Cost		$ 746,000,000
EPA Record Of Decision, Clean-Up Costs, Portland Harbor		$ 1,050,000,000.00
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		$ 44,800,000.0
Gasco Groundwater Source Control construction costs		19,000,000.0
Gasco Groundwater Source Control Allocated Insurance Proceeds To Constructed Water Treatment Station		2,500,000
Front Street liability for studies and design costs		1,100,000
SRRM pre-tax regulatory disallowance		15,000,000
Interest Income On Deferred Environmental Expenses, 2015 Order		5.3
Deferred environmental charges		95,000,000
Interest disallowed on SRRM regulatory disallowance		2,800,000
Deferred remediation expense to be collected		9,000,000
Annual tariff rider collection		5,000,000
Environmental Settlement Insurance Proceeds Received To Date		150,000,000
Annual insurance proceeds to appy against remediation costs		5,000,000
Insurance proceeds applied to prudently incurred remediation costs		63,200,000
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		14,475,000	$ 16,931,000
Environmental Noncurrent Liabilities		105,268,000	108,095,000
Regulatory Assets, Current		42,362,000	69,178,000
Regulatory Assets, Noncurrent		357,530,000	370,711,000
Annual Insurance Proceeds to Apply Against Remediation Costs		5,000,000
Total Annual Remediation Expense and Interest		$ 10,000,000
Minimum [Member]
Environmental Activity [Line Items]
Loss Contingency Range Of Possible Loss Portland Harbor		-0.3
Portland Harbor EPA Clean-Up Present Value Cost		$ (0.30)
EPA Record Of Decision, Clean-Up Costs, Portland Harbor		(0.30)
Front Street Remedial Costs		$ 7,600,000
Maximum [Member]
Environmental Activity [Line Items]
Loss Contingency Range Of Possible Loss Portland Harbor		0.5
Portland Harbor EPA Clean-Up Present Value Cost		$ 0.50
EPA Record Of Decision, Clean-Up Costs, Portland Harbor		0.50
Front Street Remedial Costs		12,900,000
Gasco Siltronic Sediments [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		869,000	2,229,000
Environmental Noncurrent Liabilities		43,972,000	42,641,000
Portland Harbor Other [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		1,970,000	1,972,000
Environmental Noncurrent Liabilities		4,148,000	5,073,000
Gasco Upland [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		10,657,000	11,550,000
Environmental Noncurrent Liabilities		49,183,000	52,454,000
Central Service Center [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		73,000	25,000
Environmental Noncurrent Liabilities		0	0
Front Street [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		906,000	1,155,000
Environmental Noncurrent Liabilities		7,786,000	7,748,000
Oregon Steel Mills [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities		0	0
Environmental Noncurrent Liabilities		179,000	179,000
Environmental Restoration Costs [Member]
Environmental Activity [Line Items]
Environmental Regulatory Deferred Cost and Interest	[1]	53,039,000	79,505,000
Environmental Regulatory Accrued Site Liabilities	[2]	119,443,000	125,026,000
Environmental Regulatory Insurance Proceeds and Interest		(98,523,000)	(118,677,000)
Environmental Regulatory Table [Abstract]
Environmental Regulatory Assets Noncurrent	[1]	73,959,000	85,854,000
Accrual for Environmental Loss Contingencies [Abstract]
Regulatory Assets, Current	[3]	9,989,000	9,270,000
Regulatory Assets, Noncurrent	[3]	63,970,000	$ 76,584,000
Environmental Restoration Costs [Member] | Front Street [Member]
Environmental Activity [Line Items]
Environmental Regulatory Accrued Site Liabilities		$ 0

[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 all sites, 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.