Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory Assets	$ 150,455	$ 164,899
Regulatory Liabilities [Line Items]
Regulatory Liabilities	159,786	29,083
Deferred Fuel Savings [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	4,229	6,016
Elm Road Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	931	3,473
Income Taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	131,689	1,302
Non-ARO Removal Cost [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	18,536	16,415
Purchased Gas Adjustment [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,404	894
Renewable Energy Credits [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	454	339
Other Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	2,543	644
Asset Retirement Obligation [Member]
Regulatory Assets [Line Items]
Regulatory Assets	8,328	5,563
Conservation Costs Regulatory Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	222	444
Debt Related Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	9,749	10,211
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets	41,958	49,281
Tax Recovery AFUDC Equity [Member]
Regulatory Assets [Line Items]
Regulatory Assets	5,497	9,403
Unfunded Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	84,342	86,475
Other Regulatory Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 359	$ 3,522