BENEFIT PLANS: Defined Benefit Plan, Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plan
Defined Benefit Plan Disclosure
Service cost		$ 33,164	$ 25,292	$ 31,357
Interest cost		35,171	35,415	31,830
Expected return on plan assets		(42,310)	(42,289)	(35,755)
Amortization of net loss		13,927	3,911	17,118
Amortization of prior service cost		221	347	347
Net periodic benefit cost		40,173	22,676	44,897
Adjustments due to the effects of regulation	[1]	(21,173)	12,124	(9,013)
Net periodic benefit cost recognized for financial reporting		19,000	34,800	35,884
Senior Management Security Plan
Defined Benefit Plan Disclosure
Service cost		1,689	1,645	2,178
Interest cost		3,868	3,856	3,258
Amortization of net loss		4,195	2,618	2,840
Amortization of prior service cost		185	220	212
Net periodic benefit cost		9,937	8,339	8,488
Net periodic benefit cost recognized for financial reporting		$ 9,937	$ 8,339	$ 8,488

[1] Net periodic benefit costs for the pension plan are recognized for financial reporting based upon the authorization of each regulatory jurisdiction in which Idaho Power operates. Under IPUC order, income statement recognition of pension plan costs is deferred until costs are recovered through rates.