SEGMENT OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 segment	Mar. 31, 2012
Segment Reporting Information
Number of reportable segments	5
External revenues	$ 1,678.2	$ 1,247.9
Depreciation and amortization expense	60.9	62.1
Earnings from equity method investments	22.3	21.1
Miscellaneous income	5.7	2.4
Interest expense	29.3	30.4
Provision (benefit) for income taxes	109.6	47.4
Net income (loss) from continuing operations	182.2	98.8
Discontinued operations, net of tax	6.1	0.9
Preferred stock dividends of subsidiary	(0.8)	(0.8)
Net income (loss) attributed to common shareholders	187.5	98.9
ATC
Segment Reporting Information
Equity method investment, ownership interest (as a percent)	34.00%
Earnings from equity method investments	21.7	20.8
Holding Company and Other
Segment Reporting Information
Discontinued operations, net of tax	6.0	1.9
Reconciling Eliminations
Segment Reporting Information
Intersegment revenues	(2.6)	(2.6)
Depreciation and amortization expense	(0.1)	(0.1)
Miscellaneous income	(3.7)	(4.2)
Interest expense	(3.7)	(4.2)
Regulated Operations
Segment Reporting Information
External revenues	1,123.8	971.0
Intersegment revenues	1.9	1.7
Depreciation and amortization expense	53.7	54.4
Earnings from equity method investments	21.7	20.8
Miscellaneous income	1.8	0.3
Interest expense	21.8	21.2
Provision (benefit) for income taxes	87.7	69.2
Net income (loss) from continuing operations	132.5	117.0
Preferred stock dividends of subsidiary	(0.8)	(0.8)
Net income (loss) attributed to common shareholders	131.7	116.2
Regulated Operations | Natural Gas Utility
Segment Reporting Information
External revenues	792.0	664.0
Intersegment revenues	1.9	1.7
Depreciation and amortization expense	32.2	32.4
Miscellaneous income	0.2	0.2
Interest expense	12.7	12.0
Provision (benefit) for income taxes	63.3	51.5
Net income (loss) from continuing operations	89.8	78.7
Preferred stock dividends of subsidiary	(0.1)	(0.1)
Net income (loss) attributed to common shareholders	89.7	78.6
Regulated Operations | Electric Utility
Segment Reporting Information
External revenues	331.8	307.0
Depreciation and amortization expense	21.5	22.0
Miscellaneous income	1.6	0.1
Interest expense	9.1	9.2
Provision (benefit) for income taxes	16.1	10.2
Net income (loss) from continuing operations	29.3	25.0
Preferred stock dividends of subsidiary	(0.7)	(0.7)
Net income (loss) attributed to common shareholders	28.6	24.3
Regulated Operations | Electric Transmission Investment
Segment Reporting Information
Earnings from equity method investments	21.7	20.8
Provision (benefit) for income taxes	8.3	7.5
Net income (loss) from continuing operations	13.4	13.3
Net income (loss) attributed to common shareholders	13.4	13.3
Nonutility and Nonregulated Operations | Integrys Energy Services
Segment Reporting Information
External revenues	545.4	269.4
Intersegment revenues	0.3	0.2
Depreciation and amortization expense	2.7	2.3
Earnings from equity method investments	0.2	0.1
Miscellaneous income	0.4	0.6
Interest expense	0.5	0.5
Provision (benefit) for income taxes	27.3	(12.3)
Net income (loss) from continuing operations	51.3	(19.1)
Discontinued operations, net of tax	0.1	(1.0)
Net income (loss) attributed to common shareholders	51.4	(20.1)
Nonutility and Nonregulated Operations | Holding Company and Other
Segment Reporting Information
External revenues	9.0	7.5
Intersegment revenues	0.4	0.7
Depreciation and amortization expense	4.6	5.5
Earnings from equity method investments	0.4	0.2
Miscellaneous income	7.2	5.7
Interest expense	10.7	12.9
Provision (benefit) for income taxes	(5.4)	(9.5)
Net income (loss) from continuing operations	(1.6)	0.9
Discontinued operations, net of tax	6.0	1.9
Net income (loss) attributed to common shareholders	$ 4.4	$ 2.8