Environmental Matters - Regulatory Deferred Assets (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Site Contingency [Line Items]
Remediation Non-recovery Percentage	3.32%	3.32%	3.32%
Regulatory assets	$ 41,241	$ 45,781	$ 49,504
Long-term regulatory assets	333,917	356,608	345,352
Annual tariff rider collection	5,000
Annual insurance proceeds to apply against remediation costs	5,000
Total annual remediation expense and interest	10,000
Environmental Restoration Costs
Site Contingency [Line Items]
Deferred costs and interest	40,578	45,546	52,888
Accrued site liabilities	116,150	126,950	117,388
Insurance proceeds and interest	(87,631)	(94,170)	(100,575)
Regulatory Assets	69,097	78,326	69,701
Regulatory assets	5,633	6,198	6,362
Long-term regulatory assets	$ 63,464	$ 72,128	$ 63,339
OREGON
Site Contingency [Line Items]
Remediation Recovery Percentage	96.68%	96.68%	96.68%
Front Street [Member] | Environmental Restoration Costs
Site Contingency [Line Items]
Accrued site liabilities	$ 400	$ 300	$ 300