Contingencies (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 claims	Dec. 31, 2012 claims	Dec. 31, 1997	Dec. 31, 2013	Jun. 30, 2013 claims	Sep. 30, 2008	Sep. 30, 2000	Dec. 31, 1993
Loss Contingencies [Line Items]
Investment in Trojan								87.00%
Regulatory asset related to Trojan							$ 5,000,000
Refund to customers for Trojan Investment including interest						33,100,000
Class action damages sought	260,000,000
Loss Contingency, Claims Settled, Number		2
contracts	119
Site Contingency, Names of Other Potentially Responsible Parties	100		69
Low estimate of range of cost of Portland Harbor cleanup in total	169,000,000
Upper estimated range of total cost of Portland harbor cleanup	1,800,000,000
Loss Contingency, Range of Possible Loss, Minimum	3,000,000
Loss Contingency, Range of Possible Loss, Maximum	8,000,000
Loss Contingency, Estimate of Possible Loss				3,000,000
regulatory asset for recovery of loss contingencies	3,000,000
Jointly Owned Utility Plant, Proportionate Ownership Share of Colstrip	20.00%
Potential Income Tax Liability related to State Tax Court Ruling	7,000,000
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					36
Loss Contingency, Pending Claims, Number					39
projects added to plaintiff's suit	2
projects encompassed by suit	40