REVENUES: (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		$ 335,402	$ 289,573
Alternative revenue programs and other revenues		14,369	19,888
Derivative revenues
Disaggregation of Revenue [Line Items]
Alternative revenue programs and other revenues		2,718	5,983
Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		267,703	264,525
Idaho fixed cost adjustment mechanism
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	[1]	(11,651)	(13,905)
Alternative revenue programs and other revenues		11,651	13,905
Wholesale energy sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		47,215	13,766
Transmission Service Agreement
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		15,728	11,698
Energy efficiency program revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		10,112	7,597
Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		6,295	5,892
Residential Retail Revenue | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	[1]	150,219	146,683
Residential Retail Revenue | Idaho fixed cost adjustment mechanism
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		11,309	13,543
Commercial Retail Revenue | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	[1]	73,106	74,227
Commercial Retail Revenue | Idaho fixed cost adjustment mechanism
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		342	362
Industrial Retail Revenue | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		45,498	45,792
Irrigation Retail Revenue | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		999	407
Deferred revenue-AFUDC | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	[2]	(2,119)	$ (2,584)
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.