GOODWILL AND OTHER INTANGIBLE ASSETS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended								12 Months Ended
	Mar. 31, 2009	Dec. 31, 2011	Dec. 31, 2009	Dec. 31, 2011 Natural Gas Utility	Dec. 31, 2010 Natural Gas Utility	Dec. 31, 2009 Natural Gas Utility	Dec. 31, 2011 Integrys Energy Services	Dec. 31, 2010 Integrys Energy Services	Dec. 31, 2009 Integrys Energy Services	Dec. 31, 2011 Holding Company and Other
Changes in the gross carrying amount of goodwill and the accumulated impairment losses
Balance at the beginning of the period, Gross goodwill		$ 940.1		$ 933.5	$ 933.5	$ 933.5	$ 6.6	$ 6.6	$ 6.6
Balance at the beginning of the period, Accumulated impairment losses		(297.6)		(297.6)	(297.6)	(297.6)
Balance at the beginning of the period, Net goodwill		642.5		635.9	635.9	635.9	6.6	6.6	6.6
Goodwill acquired		15.9								15.9
Balance at the end of the period, Gross goodwill		956.0	940.1	933.5	933.5	933.5	6.6	6.6	6.6	15.9
Balance at the end of the period, Accumulated impairment losses		(297.6)	(297.6)	(297.6)	(297.6)	(297.6)
Balance at the end of the period, Net goodwill		658.4	65.2	635.9	635.9	635.9	6.6	6.6	6.6	15.9
Noncash goodwill impairment loss	291.1		291.1
Noncash goodwill impairment loss after tax	$ 248.8