Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Regulatory Assets [Line Items]
Regulatory Assets	$ 213,182	$ 122,817
Regulatory Liabilities [Line Items]
Regulatory Liabilities	20,463	23,772
Elm Road [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	3,649	4,652
Conservation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0	776
Income Taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	2,774	3,276
Non-ARO removal cost [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	13,116	12,285
Renewable Energy Credits [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	331	2,352
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	593	431
Environmental Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	584	1,123
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets	42,356	19,230
Tax recovery related to AFUDC equity [Member]
Regulatory Assets [Line Items]
Regulatory Assets	4,400	4,400
Asset Retirement Obligation [Member]
Regulatory Assets [Line Items]
Regulatory Assets	5,847	5,634
Unfunded Pension and Other Postretirement Liability [Member]
Regulatory Assets [Line Items]
Regulatory Assets	148,467	78,659
Debt Related Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	4,662	5,137
Pension and OPRB Costs
Regulatory Assets [Line Items]
Regulatory Assets	2,468	5,090
Conservation Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	571	0
Medicare Part D Subsidy [Member]
Regulatory Assets [Line Items]
Regulatory Assets	2,851	2,851
Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 976	$ 693