SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 segment	Mar. 31, 2013
Segment reporting information
Number of reportable segments	5
Revenues	$ 2,924.9	$ 1,678.2
Depreciation and amortization expense	71.3	60.9
Earnings from equity method investments	22.9	22.3
Miscellaneous income	6.0	5.7
Interest expense	39.1	29.3
Provision (benefit) for income taxes	89.8	109.6
Net income (loss) from continuing operations	153.2	182.2
Discontinued operations, net of tax	(0.1)	6.1
Preferred stock dividends of subsidiary	(0.8)	(0.8)
Noncontrolling interest in subsidiaries	0.1	0
Net income attributed to common shareholders	152.4	187.5
ATC
Segment reporting information
Equity method investment, ownership interest (as a percent)	34.00%
Earnings from equity method investments	22.5	21.7
Intersegment revenues
Segment reporting information
Revenues	0	0
Holding Company and Other
Segment reporting information
Discontinued operations, net of tax		6.0
Reconciling Eliminations
Segment reporting information
Revenues	0	0
Depreciation and amortization expense	(0.1)	(0.1)
Earnings from equity method investments	0	0
Miscellaneous income	(3.5)	(3.7)
Interest expense	(3.5)	(3.7)
Provision (benefit) for income taxes	0	0
Net income (loss) from continuing operations	0	0
Discontinued operations, net of tax	0	0
Preferred stock dividends of subsidiary	0	0
Noncontrolling interest in subsidiaries	0
Net income attributed to common shareholders	0	0
Reconciling Eliminations | Intersegment revenues
Segment reporting information
Revenues	(7.5)	(2.6)
Regulated operations
Segment reporting information
Revenues	1,616.7	1,123.8
Depreciation and amortization expense	62.0	53.7
Earnings from equity method investments	22.5	21.7
Miscellaneous income	3.8	1.8
Interest expense	25.1	21.8
Provision (benefit) for income taxes	93.6	87.7
Net income (loss) from continuing operations	144.7	132.5
Discontinued operations, net of tax	0	0
Preferred stock dividends of subsidiary	(0.8)	(0.8)
Noncontrolling interest in subsidiaries	0
Net income attributed to common shareholders	143.9	131.7
Regulated operations | Intersegment revenues
Segment reporting information
Revenues	4.5	1.9
Regulated operations | Natural Gas Utility
Segment reporting information
Revenues	1,267.5	792.0
Depreciation and amortization expense	36.4	32.2
Earnings from equity method investments	0	0
Miscellaneous income	0.3	0.2
Interest expense	13.4	12.7
Provision (benefit) for income taxes	66.7	63.3
Net income (loss) from continuing operations	99.2	89.8
Discontinued operations, net of tax	0	0
Preferred stock dividends of subsidiary	(0.1)	(0.1)
Noncontrolling interest in subsidiaries	0
Net income attributed to common shareholders	99.1	89.7
Regulated operations | Natural Gas Utility | Intersegment revenues
Segment reporting information
Revenues	4.5	1.9
Regulated operations | Electric Utility
Segment reporting information
Revenues	349.2	331.8
Depreciation and amortization expense	25.6	21.5
Earnings from equity method investments	0	0
Miscellaneous income	3.5	1.6
Interest expense	11.7	9.1
Provision (benefit) for income taxes	18.1	16.1
Net income (loss) from continuing operations	31.8	29.3
Discontinued operations, net of tax	0	0
Preferred stock dividends of subsidiary	(0.7)	(0.7)
Noncontrolling interest in subsidiaries	0
Net income attributed to common shareholders	31.1	28.6
Regulated operations | Electric Utility | Intersegment revenues
Segment reporting information
Revenues	0	0
Regulated operations | Electric Transmission Investment
Segment reporting information
Revenues	0	0
Depreciation and amortization expense	0	0
Earnings from equity method investments	22.5	21.7
Miscellaneous income	0	0
Interest expense	0	0
Provision (benefit) for income taxes	8.8	8.3
Net income (loss) from continuing operations	13.7	13.4
Discontinued operations, net of tax	0	0
Preferred stock dividends of subsidiary	0	0
Noncontrolling interest in subsidiaries	0
Net income attributed to common shareholders	13.7	13.4
Regulated operations | Electric Transmission Investment | Intersegment revenues
Segment reporting information
Revenues	0	0
Nonutility and nonregulated operations | Integrys Energy Services
Segment reporting information
Revenues	1,289.6	545.4
Depreciation and amortization expense	2.9	2.7
Earnings from equity method investments	0.1	0.2
Miscellaneous income	0.3	0.4
Interest expense	0.5	0.5
Provision (benefit) for income taxes	5.9	27.3
Net income (loss) from continuing operations	10.9	51.3
Discontinued operations, net of tax	(0.1)	0.1
Preferred stock dividends of subsidiary	0	0
Noncontrolling interest in subsidiaries	0
Net income attributed to common shareholders	10.8	51.4
Nonutility and nonregulated operations | Integrys Energy Services | Intersegment revenues
Segment reporting information
Revenues	2.6	0.3
Nonutility and nonregulated operations | Holding Company and Other
Segment reporting information
Revenues	18.6	9.0
Depreciation and amortization expense	6.5	4.6
Earnings from equity method investments	0.3	0.4
Miscellaneous income	5.4	7.2
Interest expense	17.0	10.7
Provision (benefit) for income taxes	(9.7)	(5.4)
Net income (loss) from continuing operations	(2.4)	(1.6)
Discontinued operations, net of tax	0	6.0
Preferred stock dividends of subsidiary	0	0
Noncontrolling interest in subsidiaries	0.1
Net income attributed to common shareholders	(2.3)	4.4
Nonutility and nonregulated operations | Holding Company and Other | Intersegment revenues
Segment reporting information
Revenues	$ 0.4	$ 0.4