Discontinued Operations Summary of Cash Proceeds from Sale of Discontinued Operations (Details) (USD $)	6 Months Ended
	Jun. 30, 2014 M	Jun. 30, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contract price	$ 335,000,000
Closing adjustments	4,402,000
Gross proceeds from sale (1)	339,402,000
Cash sold in the transaction	(95,932,000)
Avista Corp. portion of proceeds held in escrow	(13,567,000)
Proceeds from Divestiture of Businesses, Net of Cash Divested	229,903,000	0
Repayment of long-term borrowings under committed line of credit	(46,000,000)	(5,000,000)
Payment to option holders and redeemable noncontrolling interests	(20,871,000)	0
Payments to Noncontrolling Interests	(54,179,000)	0
Transaction expenses withheld from proceeds	(5,390,000)
Net proceeds to Avista Capital at transaction closing	205,395,000
Business Disposition Estimated Tax Payments	(85,756,000)
Total net proceeds related to sales transaction	133,206,000
Number of Months Receivable Held in Escrow	15
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	68,100,000
Ecova [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Repayment of long-term borrowings under committed line of credit	(40,000,000)
Indemnification Agreement [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Escrow Receivable	16,750,000
Escrow Percentage of Contract Price	5.00%
Working Capital Escrow Adjustment [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Escrow Receivable	$ 1,500,000