Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Regulatory Assets [Line Items]
Regulatory Assets	$ 157,737	$ 165,183
Regulatory Liabilities [Line Items]
Regulatory Liabilities	30,300	22,715
Debt make-whole premium	0	0	$ 6,757
Conservation Costs Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	231	680
Deferred Fuel Savings [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	9,515	755
Elm Road Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	643	1,497
Income Taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,559	1,794
Non-ARO Removal Cost [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	17,137	16,129
Renewable Energy Credits [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	327	753
Other Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	888	1,107
Asset Retirement Obligation [Member]
Regulatory Assets [Line Items]
Regulatory Assets	4,849	4,532
Debt Related Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	10,672	11,133
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets	54,083	54,998
Environmental Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	368	700
Tax Recovery AFUDC Equity [Member]
Regulatory Assets [Line Items]
Regulatory Assets	8,950	8,821
Unfunded Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	78,181	84,551
Other Regulatory Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 634	$ 448