Environmental Matters (Details) $ in Thousands	1 Months Ended				9 Months Ended
	Jul. 31, 2017 USD ($)	Jan. 31, 2017 USD ($) party	Jan. 31, 2016 USD ($)	May 31, 2012	Sep. 30, 2017 USD ($) property defendant party	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Feb. 28, 2015 USD ($)
Site Contingency [Line Items]
Remediation recovery percentage in Oregon					96.68%
Number of remediation projects | property					2
Number of other parties seeking remedial costs | party		29
NRD liability claim yakama nation		$ 300
SRRM pre-tax regulatory disallowance								$ 15,000
Deferred environmental charges								$ 95,000
Total SRRM disallowance from February 2016 order			$ 3,300
Accrual for environmental loss contingencies, discount rate					1.00%
Accrual for environmental loss contingencies, annual amortization rate					0.20
Annual tariff rider collection					$ 5,000
Environmental settlement insurance proceeds received to date					$ 150,000
Percentage of environmental insurance proceeds applied to deferred costs					33.00%
Percentage of environmental insurance proceeds applied to remediation costs					67.00%
Annual insurance proceeds to appy against remediation costs					$ 5,000
Site contingency, recovery from third party of environmental cost, recognition term					20 years
Insurance proceeds applied to prudently incurred remediation costs					$ 63,200
Annual insurance proceeds to apply against remediation costs					5,000
Total annual remediation expense and interest					$ 10,000
Number of other third-party defendants | defendant					10
Environmental Restoration Costs
Site Contingency [Line Items]
Accrued site liabilities					$ 117,388	$ 119,443	$ 117,202
Front Street site | Environmental Restoration Costs
Site Contingency [Line Items]
Accrued site liabilities					$ 300
OREGON
Site Contingency [Line Items]
Remediation recovery percentage in Oregon					96.68%
Portland Harbor
Site Contingency [Line Items]
Number of potentially responsible parties (more than) | party					100
EPA final feasibility study and proposed remediation plan, clean-up costs, portland harbor		$ 1,050,000
Number of potentially responsible parties					more than 100
Portland Harbor | Minimum
Site Contingency [Line Items]
Percentage of clean-up costs		(30.00%)
Portland Harbor | Maximum
Site Contingency [Line Items]
Percentage of clean-up costs		50.00%
Gasco/Siltronic Sediments
Site Contingency [Line Items]
Loss contingency, range of possible loss gasco siltronic				$44.7 million to $350 million
Environmental project liability					$ 44,700
Gasco/Siltronic Sediments | Minimum
Site Contingency [Line Items]
Loss contingency, range of possible loss, portion not accrued					44,900
Gasco/Siltronic Sediments | Maximum
Site Contingency [Line Items]
Loss contingency, range of possible loss, portion not accrued					$ 350,000
Front Street site
Site Contingency [Line Items]
Undiscounted environmental remediation expense	$ 10,500
Additional studies and design costs	$ 1,100
Remediation non-recovery percentage for Oregon					3.32%
Yakama Nation
Site Contingency [Line Items]
Number of potentially responsible parties (more than) | party					30