GOODWILL (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 30, 2014	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Changes in the gross carrying amount of goodwill and accumulated impairment losses
Balance at the beginning of the period, gross goodwill				$ 959.7
Balance at the beginning of the period, accumulated impairment losses				(297.6)
Balance at the beginning of the period, net goodwill				662.1
Rounding adjustment				0
Goodwill impairment loss	0	(6.7)	0	(6.7)	0
Balance at the end of the period, gross goodwill		959.8		959.8
Balance at the end of the period, accumulated impairment losses		(304.4)		(304.4)
Balance at the end of the period, net goodwill		655.4		655.4
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
Rounding adjustment				(0.1)
Goodwill impairment loss				0
Balance at the end of the period, gross goodwill		933.5		933.5
Balance at the end of the period, accumulated impairment losses		(297.7)		(297.7)
Balance at the end of the period, net goodwill		635.8		635.8
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
Rounding adjustment				0.1
Goodwill impairment loss		(6.7)		(6.7)
Balance at the end of the period, gross goodwill		6.7		6.7
Balance at the end of the period, accumulated impairment losses		(6.7)		(6.7)
Balance at the end of the period, net goodwill		0		0
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				19.6
Balance at the beginning of the period, accumulated impairment losses				0
Balance at the beginning of the period, net goodwill				19.6
Rounding adjustment				0
Goodwill impairment loss				0
Balance at the end of the period, gross goodwill		19.6		19.6
Balance at the end of the period, accumulated impairment losses		0		0
Balance at the end of the period, net goodwill		$ 19.6		$ 19.6