Business Acquisitions Business Acquisitions (Details) $ / shares in Units, $ in Thousands			9 Months Ended	12 Months Ended		30 Months Ended
	Oct. 01, 2014 $ / shares shares	Jul. 01, 2014 USD ($) $ / shares shares	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) employee d	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) employee	Dec. 31, 2013 USD ($)
Business Acquisition [Line Items]
Purchase accounting adjustments				$ (304)	$ 112
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	1,427	4,500,014
Business Combination Equity Consideration Share Price | $ / shares		$ 32.46
Gross contract price		$ 170,000
Business Combination Number of Trading Days to Price Equity Consideration Share Price | d				10
Business Acquisition, Share Price | $ / shares	$ 30.71	$ 33.35
Goodwill		$ 52,426		$ 57,672	57,976	$ 57,672	$ 76,257
Alaska Electric Light & Power [Member]
Business Acquisition [Line Items]
Purchase accounting adjustments			$ (300)	$ (304)	0
Number of Customers | employee				17,000		17,000
Goodwill			$ 52,400	$ 52,426	$ 52,730	$ 52,426	$ 0
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