Fair Value of Financial Assets and Liabilities, Derivative Assets and Liabilities at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current derivative liabilities [Abstract]
Other current liabilities	$ 13,178	$ 11,794
Fair Value Measured on a Recurring Basis [Member]
Current derivative liabilities [Abstract]
Natural gas commodity	0	11	[1]
Fair Value Measured on a Recurring Basis [Member] | Level 1 [Member]
Current derivative liabilities [Abstract]
Natural gas commodity	0	0
Fair Value Measured on a Recurring Basis [Member] | Level 2 [Member]
Current derivative liabilities [Abstract]
Natural gas commodity	0	11
Fair Value Measured on a Recurring Basis [Member] | Level 3 [Member]
Current derivative liabilities [Abstract]
Natural gas commodity	0	0
Fair Value Measured on a Recurring Basis [Member] | Fair Value Total [Member]
Current derivative liabilities [Abstract]
Natural gas commodity	0	11
Other current liabilities		11,800
Fair Value Measured on a Recurring Basis [Member] | Counterparty Netting [Member]
Current derivative liabilities [Abstract]
Natural gas commodity	$ 0	$ 0	[2]

[1] Included in other current liabilities balance of $11.8 million at Dec. 31, 2012 in the consolidated balance sheets.

[2] NSP-Wisconsin nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2012. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.