Business Acquisitions Business Acquisitions (Details) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended	42 Months Ended
	Jun. 30, 2015 USD ($)	Oct. 01, 2014 shares	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) employee	Dec. 31, 2014 USD ($)	Jul. 01, 2014 USD ($) $ / shares
Business Acquisition [Line Items]
Purchase accounting adjustments			$ (304)
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Gross Accounts Receivable						$ 3,928
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares | shares		4,501,441
Business Combination Equity Consideration Share Price | $ / shares						$ 32.46
Gross contract price						$ 170,000
Goodwill			57,672	$ 57,672	$ 57,976	$ 52,426
Alaska Electric Light & Power [Member]
Business Acquisition [Line Items]
Purchase accounting adjustments	$ (300)		(304)
Number of Customers | employee				17,000
Goodwill			$ 52,426	$ 52,426	$ 52,730
Avista Utilities [Member]
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs				3,000
Avista Utilities [Member] | Acquisition-related Costs [Member]
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs				$ 400