Fair Value of Financial Assets and Liabilities, Derivative Assets and Liabilities at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Current derivative liabilities [Abstract]
Natural gas commodity	$ 11	[1]	$ 2,514	[1]
Other current liabilities	11,794		13,907
Level 1 [Member]
Current derivative liabilities [Abstract]
Natural gas commodity	0		418
Level 2 [Member]
Current derivative liabilities [Abstract]
Natural gas commodity	11		2,096
Level 3 [Member]
Current derivative liabilities [Abstract]
Natural gas commodity	0		0
Fair Value Total [Member]
Current derivative liabilities [Abstract]
Natural gas commodity	11		2,514
Counterparty Netting [Member]
Current derivative liabilities [Abstract]
Natural gas commodity	$ 0	[2]	$ 0	[2]

[1] Included in other current liabilities balance of $11.8 million and $13.9 million at Dec. 31, 2012 and 2011, respectively, in the consolidated balance sheets.

[2] The accounting guidance for derivatives and hedging permits the netting of receivables and payables for derivatives and related collateral amounts when a legally enforceable master netting agreement exists between NSP-Wisconsin and a counterparty. A master netting agreement is an agreement between two parties who have multiple contracts with each other that provides for the net settlement of all contracts in the event of default on or termination of any one contract.