REGULATORY MATTERS: Revenue Sharing Under January 2010 and December 2011 Idaho Settlement Agreements (Details) (USD $)								12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Revenue Sharing Under January 2010 and December 2011 Idaho Settlement Agreements [Member] Settlement agreement sharing [Member]		Dec. 31, 2011 Revenue Sharing Under January 2010 and December 2011 Idaho Settlement Agreements [Member] Settlement agreement sharing [Member]		Dec. 27, 2011 December 2011 Idaho Settlement Agreement	Dec. 31, 2012 Pension Plan	Dec. 31, 2011 Pension Plan
Revenue Sharing Under January 2010 and December 2011 Idaho Settlement Agreements [Line Items]
Authorized Return on Equity in Rate Case, Maximum							10.50%
Regulatory Liabilities	$ 385,639,000	$ 361,795,000	$ 7,151,000	[1],[2]	$ 27,099,000	[1]
Additional Pension Amortization As Result Of Idaho Settlement Stipulation								$ 14,600,000	$ 20,300,000

[1] These items are discussed in more detail in this Note 3.

[2] Earning a return includes either interest or a return on the investment as a component of rate base at the allowed rate of return.