GOODWILL ROLLFORWARD (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
Changes in the gross carrying amount of goodwill and accumulated impairment losses
Balance at the beginning of the period, Gross goodwill	$ 955.9
Balance at the beginning of the period, Accumulated impairment losses	(297.6)
Balance at the beginning of the period, Net goodwill	658.3
Adjustment to ITF patents/intellectual property	3.8
Balance at the end of the period, Gross goodwill	959.7
Balance at the end of the period, Accumulated impairment losses	(297.6)
Balance at the end of the period, Net goodwill	662.1
Goodwill impairment loss	0
Natural Gas Utility
Changes in the gross carrying amount of goodwill and accumulated impairment losses
Balance at the beginning of the period, Gross goodwill	933.5
Balance at the beginning of the period, Accumulated impairment losses	(297.6)
Balance at the beginning of the period, Net goodwill	635.9
Adjustment to ITF patents/intellectual property	0
Balance at the end of the period, Gross goodwill	933.5
Balance at the end of the period, Accumulated impairment losses	(297.6)
Balance at the end of the period, Net goodwill	635.9
Integrys Energy Services
Changes in the gross carrying amount of goodwill and accumulated impairment losses
Balance at the beginning of the period, Gross goodwill	6.6
Balance at the beginning of the period, Accumulated impairment losses	0
Balance at the beginning of the period, Net goodwill	6.6
Adjustment to ITF patents/intellectual property	0
Balance at the end of the period, Gross goodwill	6.6
Balance at the end of the period, Accumulated impairment losses	0
Balance at the end of the period, Net goodwill	6.6
Holding Company and Other
Changes in the gross carrying amount of goodwill and accumulated impairment losses
Balance at the beginning of the period, Gross goodwill	15.8
Balance at the beginning of the period, Accumulated impairment losses	0
Balance at the beginning of the period, Net goodwill	15.8
Adjustment to ITF patents/intellectual property	3.8
Balance at the end of the period, Gross goodwill	19.6
Balance at the end of the period, Accumulated impairment losses	0
Balance at the end of the period, Net goodwill	$ 19.6