SHORT-TERM DEBT AND LINES OF CREDIT - REVOLVING CREDIT FACILITIES (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Short-term borrowings
Total short-term credit capacity	$ 1,600.0	$ 1,610.0
Letters of credit issued inside credit facilities	29.4	52.4
Short-term borrowings outstanding	392.5	326.0
Available capacity under existing agreements	1,178.1	1,231.6
Revolving credit facility maturing on May 17, 2014 | Integrys Energy Group, Inc.
Short-term borrowings
Total short-term credit capacity	0	275.0
Revolving credit facility maturing on May 17, 2014 | WPS
Short-term borrowings
Total short-term credit capacity	0	135.0
Revolving credit facility maturing on May 7, 2015 | WPS
Short-term borrowings
Total short-term credit capacity	135.0	0
Revolving credit facility maturing on May 17, 2016 | Integrys Energy Group, Inc.
Short-term borrowings
Total short-term credit capacity	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
Revolving credit facility maturing on May 8, 2019 | Integrys Energy Group, Inc.
Short-term borrowings
Total short-term credit capacity	465.0	0
Commercial paper
Short-term borrowings
Short-term borrowings outstanding	$ 392.5	$ 326.0