Contingencies (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2008	Dec. 31, 1997	Sep. 30, 2008	Dec. 31, 2003	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
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
Repayments of Pollution Control Bond		5,800,000
Potential Income Tax Liability related to State Tax Court Ruling	$ 12,000,000