PROPERTY, PLANT, AND EQUIPMENT (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended				3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Winnebago Energy Center	Sep. 30, 2010 Integrys Energy Services plant	Dec. 31, 2010 Integrys Energy Services	Dec. 31, 2011 Utility Segments	Dec. 31, 2010 Utility Segments	Dec. 31, 2011 Utility Segments Construction work in progress	Dec. 31, 2010 Utility Segments Construction work in progress	Dec. 31, 2011 Utility Segments Electric Utility	Dec. 31, 2010 Utility Segments Electric Utility	Dec. 31, 2011 Utility Segments Natural Gas Utility	Dec. 31, 2010 Utility Segments Natural Gas Utility	Dec. 31, 2011 Nonutility plant	Dec. 31, 2010 Nonutility plant	Dec. 31, 2011 Nonutility plant Construction work in progress	Dec. 31, 2010 Nonutility plant Construction work in progress	Dec. 31, 2011 Nonregulated property, plant and equipment	Dec. 31, 2010 Nonregulated property, plant and equipment	Dec. 31, 2011 Nonregulated property, plant and equipment Construction work in progress	Dec. 31, 2010 Nonregulated property, plant and equipment Construction work in progress	Dec. 31, 2011 Nonregulated property, plant and equipment Integrys Energy Services energy assets	Dec. 31, 2010 Nonregulated property, plant and equipment Integrys Energy Services energy assets	Dec. 31, 2011 Nonregulated property, plant and equipment Integrys Energy Services other	Dec. 31, 2010 Nonregulated property, plant and equipment Integrys Energy Services other	Dec. 31, 2011 Nonregulated property, plant and equipment Other.	Dec. 31, 2010 Nonregulated property, plant and equipment Other.
Property, plant, and equipment
Total property, plant, and equipment							$ 7,891.1	$ 7,601.8	$ 62.0	$ 39.5	$ 3,139.7	$ 3,095.5	$ 4,751.4	$ 4,506.3	$ 130.4	$ 143.9	$ 2.7	$ 1.6	$ 128.6	$ 126.0	$ 3.0	$ 0.8	$ 100.6	$ 98.9	$ 20.0	$ 21.1	$ 8.0	$ 6.0
Less: Accumulated depreciation	3,018.7	2,900.2					2,910.1	2,794.2							68.7	70.2			39.9	35.8
Net property, plant, and equipment excluding construction work in progress							4,981.0	4,807.6							61.7	73.7			88.7	90.2
Net property, plant, and equipment	5,199.1	5,013.4					5,043.0	4,847.1							64.4	75.3			91.7	91.0
Pre-tax noncash impairment loss	$ 4.6	$ 43.2	$ 0.7	$ 4.6	$ 43.2
Number of natural gas-fired generation plants related to which impairment loss was recorded					3
Discount rate used for the impairment analysis (as a percent)				7.50%		10.00%