Environmental Matters (Details) $ in Millions	1 Months Ended		3 Months Ended
	Jul. 31, 2017 USD ($)	Jan. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) property party
Site Contingency [Line Items]
Annual tariff rider collection			$ 5.0
Annual insurance proceeds to apply against remediation costs			5.0
Total annual remediation expense and interest			$ 10.0
OREGON
Site Contingency [Line Items]
Remediation recovery percentage in Oregon			96.68%
Super Fund Site [Member]
Site Contingency [Line Items]
Number of potentially responsible parties (more than) | party			100
Number of remediation projects | property			2
Portland Harbor
Site Contingency [Line Items]
EPA final feasibility study and proposed remediation plan, clean-up costs, portland harbor		$ 1,050.0
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%
Front Street site
Site Contingency [Line Items]
Undiscounted environmental remediation expense	$ 10.5
Additional studies and design costs	$ 1.0
Remediation non-recovery percentage for Oregon			3.32%