Contingencies (Details) (USD $)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2013 claims	Sep. 30, 2013 claims	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 1997	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	260,000,000
Site Contingency, Names of Other Potentially Responsible Parties		100			69
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	36
Loss Contingency, Pending Claims, Number	39	39
Low estimate of range of cost of Portland Harbor cleanup in total	169,000,000	169,000,000
Upper estimated range of total cost of Portland harbor cleanup	1,800,000,000	1,800,000,000
Potential Income Tax Liability related to State Tax Court Ruling	7,000,000	7,000,000
Damages sought in USDOE claim				112,000,000
Loss Contingency, Damages Awarded, Value	70,000,000
Proceeds received from Trojan spent fuel legal settlement		$ 44,000,000	$ 0