Contingencies (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) claims	Dec. 31, 1997	Sep. 30, 2015 USD ($)	Jun. 30, 2013 claims	Sep. 30, 2008 USD ($)	Dec. 31, 1993
Loss Contingencies [Line Items]
investment in Trojan						87.00%
Refund to customers for Trojan Investment including interest					$ 33,000,000
Class action damages sought			$ 260,000,000
Site Contingency, Names of Other Potentially Responsible Parties	100	69
Low estimate of range of cost of Portland Harbor cleanup in total			800,000,000
Upper estimated range of total cost of Portland harbor cleanup			$ 2,400,000,000
Loss Contingency, Range of Possible Loss, Maximum	$ 1,500,000,000
Jointly Owned Utility Plant, Proportionate Ownership Share of Colstrip	20.00%
Civil Penalty Claim - Per day per violation through January 12, 2009	$ 32,500
Civil Penalty Claim - Per day per violation after January 12, 2009	$ 37,500
Claims moved to dismiss | claims				36
Loss Contingency, Pending Claims, Number | claims				39
projects added to plaintiff's suit | claims	2
projects encompassed by suit | claims	40