Business Acquisitions (Details) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended		15 Months Ended
	Oct. 01, 2014 $ / shares shares	Jul. 01, 2014 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) employee	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) employee d MW	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) employee	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 57,672		$ 57,672		$ 57,672	$ 57,976
Business Acquisition, Share Price | $ / shares	$ 30.71	$ 33.35
Operating revenues			313,649	$ 301,558	$ 1,097,471	$ 1,060,716
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
Business Combination Number of Trading Days to Price Equity Consideration Share Price | d					10
Merger Consideration - Total		$ 170,000
Representative Reimbursement Amount							4,792
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents		19,704
Business Acquisition Recognized Identifiable Assets Acquired and Liabilities Assumed Current and Long-Term Debt		38,832
Business Acquisition Contract Price Closing Adjustments		58
Business Acquisition Adjusted Contract Price		$ 150,814
Business Combination, Consideration Transferred							154,911
Alaska Energy Resources Company [Member]
Business Acquisition [Line Items]
Operating revenues			9,307	$ 9,224	$ 32,345	$ 35,319
Alaska Electric Light & Power [Member]
Business Acquisition [Line Items]
Goodwill, Purchase Accounting Adjustments					(300)
Goodwill			$ 52,400		$ 52,400		$ 52,400
Number of Customers | employee			16,676		16,676		16,676
Alaska Electric Light & Power [Member] | Power purchase agreement [Member]
Business Acquisition [Line Items]
Evaluated Power Capacity | MW					78