Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Regulatory Assets	$ 113,543	$ 229,180
Regulatory Liabilities [Line Items]
Regulatory Liabilities	33,330	24,538
Debt make-whole premium	6,757	0	0
Conservation Costs Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	455	455
Deferred Fuel Savings [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	13,386	6,163
Elm Road Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	607	0
Income Taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	2,082	2,346
Non-ARO Removal Cost [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	15,182	13,957
Renewable Energy Credits [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	574	480
Other Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,044	1,137
Asset Retirement Obligation [Member]
Regulatory Assets [Line Items]
Regulatory Assets	4,863	4,608
Debt Related Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	11,786	5,468
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets	63,893	72,328
Elm Road Regulatory Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	0	2,614
Environmental Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	920	1,154
Pension and OPRB Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	0	1,234
Tax Recovery AFUDC Equity [Member]
Regulatory Assets [Line Items]
Regulatory Assets	6,956	5,241
Unfunded Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	24,591	134,839
Other Regulatory Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 534	$ 1,694