REVENUES: (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	May 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		$ 331,298	$ 326,190	$ 620,871	$ 620,081
Alternative revenue programs and other revenues		7,401	5,578	27,289	13,651
Contract with Customer, Asset, Credit Loss Expense		1,500	2,200	1,800	2,400
Derivative revenues
Disaggregation of Revenue [Line Items]
Alternative revenue programs and other revenues		1,602	2,097	7,585	4,933
Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		298,591	295,001	563,114	565,233
Idaho fixed cost adjustment mechanism
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	[1]	(5,799)	(3,481)	(19,704)	(8,718)
Alternative revenue programs and other revenues		5,799	3,481	19,704	8,718
Wholesale energy sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		10,214	6,003	24,283	13,967
Transmission Service Agreement
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		13,205	11,965	24,600	20,824
Energy efficiency program revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		8,802	10,515	16,399	16,843
Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		6,285	6,187	12,179	11,932
Residential Retail Revenue | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	[1]	109,155	112,534	255,838	264,689
Residential Retail Revenue | Idaho fixed cost adjustment mechanism
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		5,508	3,205	19,052	8,331
Commercial Retail Revenue | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	[1]	76,965	78,982	151,191	153,260
Commercial Retail Revenue | Idaho fixed cost adjustment mechanism
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		291	276	652	387
Industrial Retail Revenue | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		48,868	49,766	94,660	95,224
Irrigation Retail Revenue | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		65,065	56,068	65,471	56,993
Deferred revenue-AFUDC | Retail revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	[2]	(1,462)	$ (2,349)	(4,046)	$ (4,933)
IPUC authorized AFUDC Collection HCC Relicensing - Gross | Idaho Power Company | Hells Canyon Complex
Disaggregation of Revenue [Line Items]
Regulatory liabilities		(8,800)		(8,800)		$ (10,700)
Energy efficiency regulatory liability
Disaggregation of Revenue [Line Items]
Regulatory liabilities		(2,600)		(2,600)
Energy efficiency regulatory asset
Disaggregation of Revenue [Line Items]
Regulatory assets		$ 6,500		$ 6,500

[1] The FCA mechanism is an alternative revenue program 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 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. Prior to the May 2018 Idaho Tax Reform Settlement Stipulation described in Note 3 - "Regulatory Matters," Idaho Power was collecting $10.7 million annually.