Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory Assets	$ 164,899	$ 157,737
Regulatory Liabilities [Line Items]
Regulatory Liabilities	29,083	30,300
Conservation Costs Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0	231
Deferred Fuel Savings [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	6,016	9,515
Elm Road Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	3,473	643
Income Taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,302	1,559
Non-ARO Removal Cost [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	16,415	17,137
Renewable Energy Credits [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	339	327
Other Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,538	888
Asset Retirement Obligation [Member]
Regulatory Assets [Line Items]
Regulatory Assets	5,563	4,849
Conservation Costs Regulatory Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	444	0
Debt Related Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	10,211	10,672
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets	49,281	54,083
Environmental Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	55	368
Tax Recovery AFUDC Equity [Member]
Regulatory Assets [Line Items]
Regulatory Assets	9,403	8,950
Unfunded Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	86,475	78,181
Other Regulatory Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 3,467	$ 634