SHORT-TERM DEBT AND LINES OF CREDIT (Details 2) (USD $) In Millions, unless otherwise specified	Sep. 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	37.4	25.5
Short-term borrowings outstanding	388.0	482.4
Available capacity under existing agreements	1,384.6	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 | WPS
Short-term borrowings
Total short-term credit capacity	200.0	0
Loan under term credit facility
Short-term borrowings
Short-term borrowings outstanding	200.0	0
Commercial paper
Short-term borrowings
Short-term borrowings outstanding	$ 188.0	$ 482.4