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SHORT-TERM DEBT AND LINES OF CREDIT - REVOLVING CREDIT FACILITIES (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2014
Dec. 31, 2013
Short-term borrowings    
Total short-term credit capacity $ 1,610.0 $ 1,610.0
Letters of credit issued inside credit facilities 70.6 52.4
Short-term borrowings outstanding 321.9 326.0
Available capacity under existing agreements 1,217.5 1,231.6
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
Commercial paper
    
Short-term borrowings    
Short-term borrowings outstanding $ 321.9 $ 326.0