Business Acquisitions (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended		3 Months Ended		12 Months Ended
	Oct. 01, 2014	Jul. 01, 2014	Mar. 31, 2015 D	Mar. 31, 2014	Dec. 31, 2014	Oct. 01, 2014	Jul. 01, 2014
Business Acquisition [Line Items]
Goodwill			$ 57,976		$ 57,976
Business Acquisition, Share Price	$ 30.71	$ 33.35				$ 30.71	$ 33.35
Operating revenues			446,490	446,578
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	1,427	4,500,014
Business Combination Equity Consideration Share Price		$ 32.46					$ 32.46
Business Combination Number of Trading Days to Price Equity Consideration Share Price			10
Merger Consideration - Total		170,000					170,000
Representative Reimbursement Amount					4,697
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents		19,704					19,704
Business Acquisition Recognized Identifiable Assets Acquired and Liabilities Assumed Current and Long-Term Debt		38,832					38,832
Business Acquisition Contract Price Closing Adjustments		58					58
Business Acquisition Adjusted Contract Price		150,814					150,814
Business Combination, Consideration Transferred		154,816
Alaska Energy Resources Company [Member]
Business Acquisition [Line Items]
Operating revenues			12,791	13,186
Alaska Electric Light & Power [Member]
Business Acquisition [Line Items]
Goodwill		52,730					52,730
Operating revenues			$ 12,774	$ 0
Number of Customers			16,452
Alaska Electric Light & Power [Member] | Power purchase agreement [Member]
Business Acquisition [Line Items]
Evaluated Power Capacity			78