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SHORT-TERM DEBT AND LINES OF CREDIT (Details 2) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Short-term borrowings    
Total short-term credit capacity $ 1,810.0 $ 1,610.0
Letters of credit issued inside credit facilities 27.8 25.5
Short-term borrowings 833.2 482.4
Available capacity under existing agreements 948.9 1,102.1
Revolving credit facility maturing on May 17, 2014 | Integrys Energy Group, Inc.
    
Short-term borrowings    
Total short-term credit capacity 275.0 275.0
Revolving credit facility maturing on May 17, 2014 | WPS
    
Short-term borrowings    
Total short-term credit capacity 135.0 135.0
Revolving credit facility maturing on May 17, 2016 | Integrys Energy Group, Inc.
    
Short-term borrowings    
Total short-term credit capacity 200.0 200.0
Revolving credit facility maturing on June 13, 2017 | Integrys Energy Group, Inc.
    
Short-term borrowings    
Total short-term credit capacity 635.0 635.0
Revolving credit facility maturing on June 13, 2017 | WPS
    
Short-term borrowings    
Total short-term credit capacity 115.0 115.0
Revolving credit facility maturing on June 13, 2017 | PGL
    
Short-term borrowings    
Total short-term credit capacity 250.0 250.0
Term credit facility maturing on December 31, 2013
    
Short-term borrowings    
Short-term borrowings 200.0 0
Accrued interest or original discount on outstanding debt   0
Term credit facility maturing on December 31, 2013 | WPS
    
Short-term borrowings    
Total short-term credit capacity 200.0 0
Commercial paper
    
Short-term borrowings    
Short-term borrowings 633.2 482.4
Accrued interest or original discount on outstanding debt $ 0.1