REGULATORY MATTERS: Level 4 (Details) (USD $)	3 Months Ended							12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Oct. 02, 2012 Idaho and Oregon General Rate Cases and Base Rate Adjustments	Jun. 29, 2012 Idaho and Oregon General Rate Cases and Base Rate Adjustments	Mar. 01, 2012 Idaho and Oregon General Rate Cases and Base Rate Adjustments	Jan. 02, 2012 Idaho and Oregon General Rate Cases and Base Rate Adjustments	Dec. 27, 2011 Settlement Stipulation - Investment Tax Credits and Idaho Sharing Mechanism	Dec. 31, 2012 Revenue Sharing Under January 2010 and December 2011 Idaho Settlement Agreements	Dec. 31, 2011 Revenue Sharing Under January 2010 and December 2011 Idaho Settlement Agreements	Apr. 15, 2013 Annual Idaho PCA Mechanism Filing	Jun. 01, 2012 Annual Idaho PCA Mechanism Filing	Jun. 01, 2013 Annual Idaho Fixed Cost Adjustment Filing	Jun. 01, 2012 Annual Idaho Fixed Cost Adjustment Filing	Apr. 13, 2013 Annual Idaho Demand-Side Managment Prudence and Cost Recovery Filings	Mar. 31, 2013 Annual Idaho Demand-Side Managment Prudence and Cost Recovery Filings
Rate Case
Authorized Rate of Return in Rate Case					7.757%	7.86%
Total Retail Rate Base						$ 2,360,000,000
Increase (Decrease) in Rates			3,000,000	58,100,000	1,800,000	34,000,000				140,400,000	43,000,000			14,500,000
Increase (Decrease) in Rate Base				335,900,000
Authorized Return on Equity in Rate Case, Minimum					9.90%		9.50%
Authorized Return on Equity in Rate Case, Mid-point							10.00%
Authorized Return on Equity in Rate Case, Maximum							10.50%
Investment Tax Credits, Maximum, in Rate Case							45,000,000
Investment Tax Credits Maximum in one Year in Rate Case							25,000,000
Period of Current ADITC Stipulation							3 years
Percentage to be Shared with Customers							75.00%
Percentage to be Shared with Entity							25.00%
Automatic adjustment of authorized return on equity in rate case, minimum							95.00%
Automatic adjustment of authorized return on equity in rate case, maximum							105.00%
Regulatory Liabilities								7,200,000	27,100,000
Reduction in Pension Regulatory Asset as a Result of Settlement Stipulation	(13,681,000)	15,586,000						14,600,000	20,300,000
Deferred Increase (Decrease) in Rates										52,500,000
Interest Rate on Regulatory Asset										1.00%
Net PCA Rate Increase with Revenue Sharing											15,900,000
Annual FCA Collection												8,900,000	10,300,000
Demand-Side Management Rider Mechanism														25,900,000
Regulatory Assets														$ 6,000,000	$ 14,000,000