REVENUES: (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		$ 270,184	$ 335,402
Alternative revenue programs and other revenues		20,304	14,369
Derivative revenues
Disaggregation of Revenue [Line Items]
Alternative revenue programs and other revenues		4,110	2,718
Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		256,416	267,703
Idaho fixed cost adjustment mechanism
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	[1]	(16,194)	(11,651)
Alternative revenue programs and other revenues		16,194	11,651
Wholesale energy sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		3,908	47,215
Transmission Service Agreement
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		10,363	15,728
Energy efficiency program revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		9,475	10,112
Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		6,216	6,295
Residential Retail Revenue | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	[1]	144,887	150,219
Residential Retail Revenue | Idaho fixed cost adjustment mechanism
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		15,709	11,309
Commercial Retail Revenue | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	[1]	69,513	73,106
Commercial Retail Revenue | Idaho fixed cost adjustment mechanism
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		484	342
Industrial Retail Revenue | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		42,760	45,498
Irrigation Retail Revenue | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		1,375	999
Deferred revenue-AFUDC | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	[2]	(2,119)	$ (2,119)
IPUC authorized AFUDC Collection HCC Relicensing - Gross | Idaho Power Company | Hells Canyon Complex
Disaggregation of Revenue [Line Items]
Regulatory liabilities		$ (8,800)

[1] The FCA mechanism is an alternative revenue program in the Idaho jurisdiction and does not represent revenue from contracts with customers.

[2] As part of its January 30, 2009, general rate case order, the IPUC is allowing Idaho Power to recover a portion of the allowance for funds used during construction (AFUDC) on construction work in progress related to the Hells Canyon Complex (HCC) relicensing process, even though the relicensing process is not yet complete and the costs have not been moved to electric plant in service. Idaho Power is collecting $8.8 million annually in the Idaho jurisdiction but is deferring revenue recognition of the amounts collected until the license is issued and the accumulated license costs approved for recovery are placed in service.