Environmental Matters (Details) - USD ($)		3 Months Ended
		Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Environmental Activity [Line Items]
Remediation recovery percentage in Oregon		96.68%
Loss Contingency Range Of Possible Loss Portland Harbor		$791 million to $2.45 billion
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		1,200,000
SRRM pre-tax regulatory disallowance		15,000,000
Deferred environmental charges		95,000,000
After-tax regulatory disallowance on insurance proceeds		5,300,000
SRRM Amortization Expenses		13,800,000
Total SRRM Disallowance from Feb 2016 Order		3,300,000
Interest disallowed on SRRM regulatory disallowance		2,800,000
SRRM Disallowance, Portion attributable to Operations and Maintenance		500,000
Deferred remediation expense to be collected		8,400,000
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		58,200,000
Environmental Regulatory Table [Abstract]
Regulatory Assets, Current		61,524,000	$ 69,178,000	$ 67,702,000
Regulatory Assets, Noncurrent		351,390,000	370,711,000	348,421,000
Accrual for Environmental Loss Contingencies [Abstract]
Accrued Environmental Loss Contingencies, Current		16,124,000	16,931,000	12,760,000
Accrued Environmental Loss Contingencies, Noncurrent		106,182,000	108,095,000	81,097,000
Minimum [Member]
Environmental Activity [Line Items]
Front Street Remedial Costs		7,600,000
Maximum [Member]
Environmental Activity [Line Items]
Front Street Remedial Costs		12,900,000
Gasco Siltronic Sediments [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Accrued Environmental Loss Contingencies, Current		2,747,000	2,229,000	1,572,000
Accrued Environmental Loss Contingencies, Noncurrent		42,079,000	42,641,000	38,379,000
Portland Harbor Other [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Accrued Environmental Loss Contingencies, Current		1,655,000	1,972,000	1,308,000
Accrued Environmental Loss Contingencies, Noncurrent		4,775,000	5,073,000	5,186,000
Gasco Upland [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Accrued Environmental Loss Contingencies, Current		10,405,000	10,599,000	8,205,000
Accrued Environmental Loss Contingencies, Noncurrent		51,070,000	52,117,000	36,833,000
Siltronic Upland [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Accrued Environmental Loss Contingencies, Current		221,000	951,000	750,000
Accrued Environmental Loss Contingencies, Noncurrent		333,000	337,000	405,000
Central Service Center [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Accrued Environmental Loss Contingencies, Current		25,000	25,000	170,000
Accrued Environmental Loss Contingencies, Noncurrent		0	0	0
Front Street [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Accrued Environmental Loss Contingencies, Current		1,071,000	1,155,000	755,000
Accrued Environmental Loss Contingencies, Noncurrent		7,746,000	7,748,000	115,000
Oregon Steel Mills [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Accrued Environmental Loss Contingencies, Current		0	0	0
Accrued Environmental Loss Contingencies, Noncurrent		179,000	179,000	179,000
Environmental Restoration Costs [Member]
Environmental Regulatory Table [Abstract]
Environmental Regulatory- deferred costs and interest	[1]	57,359,000	79,505,000	76,790,000
Environmental Regulatory accrued site liabilities		122,306,000	125,026,000	93,858,000
Environmental Regulatory insurance proceeds and interest		(102,570,000)	(118,677,000)	(120,473,000)
Environmental Regulatory Assets Noncurrent	[1]	77,095,000	85,854,000		$ 50,175,000
Regulatory Assets, Current	[2],[3]	9,096,000	9,270,000	0
Regulatory Assets, Noncurrent	[3]	$ 67,999,000	$ 76,584,000	$ 50,175,000

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

[2] 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.

[3] nvironmental 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 subject to an earnings test. See Note 13.