BENEFIT PLANS: Level 4 (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		8 Months Ended	12 Months Ended
		Apr. 28, 2016	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2016
Pension Plan
Defined Benefit Plan Disclosure
Service cost			$ 8,117	$ 8,459
Interest cost			9,423	8,820
Expected return on plan assets			(10,170)	(10,219)
Amortization of prior service cost			13	55
Amortization of net loss			3,384	3,546
Net periodic benefit cost			10,767	10,661
Adjustments due to the effects of regulation	[1]		(6,019)	(5,876)
Net periodic benefit cost recognized for financial reporting	[1]		4,748	4,785
Contributions made to the defined benefit pension plan		$ 10,000	0
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year					$ 30,000
Senior Management Security Plan
Defined Benefit Plan Disclosure
Service cost			307	422
Interest cost			1,069	967
Expected return on plan assets			0	0
Amortization of prior service cost			42	46
Amortization of net loss			883	1,049
Net periodic benefit cost			2,301	2,484
Net periodic benefit cost recognized for financial reporting			2,301	2,484
Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost			293	338
Interest cost			696	674
Expected return on plan assets			(620)	(673)
Amortization of prior service cost			7	4
Amortization of net loss			0	0
Net periodic benefit cost			376	343
Net periodic benefit cost recognized for financial reporting			$ 376	$ 343
Subsequent Event [Member] | Pension Plan
Defined Benefit Plan Disclosure
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year						$ 10,000

[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.