GOODWILL AND OTHER INTANGIBLE ASSETS – GOODWILL (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended
	Apr. 30, 2014	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Natural Gas Utility	Jun. 30, 2014 Integrys Energy Services	Sep. 30, 2014 Integrys Energy Services	Sep. 30, 2014 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				$ 959.7		$ 933.5		$ 6.6	$ 19.6
Balance at the beginning of the period, accumulated impairment losses				(297.6)		(297.6)		0	0
Balance at the beginning of the period, net goodwill				662.1		635.9		6.6	19.6
Rounding adjustment				0		(0.1)		0.1	0
Goodwill impairment loss	0	0	0	(6.7)	0	0	(6.7)	(6.7)	0
Balance at the end of the period, gross goodwill		959.8		959.8		933.5		6.7	19.6
Balance at the end of the period, accumulated impairment losses		(304.4)		(304.4)		(297.7)		(6.7)	0
Balance at the end of the period, net goodwill		$ 655.4		$ 655.4		$ 635.8		$ 0	$ 19.6