Contingencies (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2008	Dec. 31, 2007	Dec. 31, 2005	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
Site Contingency, Names of Other Potentially Responsible Parties	100			6	14	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
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
Estimated award range from USDOE claim, Low	65,000,000
Estimated award range from USDOE claim, High	75,000,000
Damages sought in USDOE claim		$ 112,000,000