SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 segment	Sep. 30, 2012
Segment Reporting Information
Number of reportable segments			5
External revenues	$ 1,129.7	$ 927.7	$ 3,923.9	$ 3,015.2
Intersegment revenues	0	0	0	0
Depreciation and amortization expense	69.6	62.9	196.0	187.6
Earnings from equity method investments	23.1	22.2	68.2	65.5
Miscellaneous income	12.1	3.1	23.3	7.2
Interest expense	33.1	29.9	91.0	90.0
Provision (benefit) for income taxes	18.0	29.6	124.3	106.6
Net income (loss) from continuing operations	39.4	74.3	217.7	224.8
Discontinued operations, net of tax	(0.6)	(8.0)	4.7	(9.2)
Preferred stock dividends of subsidiary	(0.7)	(0.7)	(2.3)	(2.3)
Noncontrolling interest in subsidiaries	0	0.1	0.1	0.1
Net income attributed to common shareholders	38.1	65.7	220.2	213.4
ATC
Segment Reporting Information
Equity method investment, ownership interest (as a percent)	34.00%		34.00%
Reconciling Eliminations
Segment Reporting Information
External revenues	0	0	0	0
Intersegment revenues	(4.9)	(4.9)	(10.6)	(10.2)
Depreciation and amortization expense	(0.1)	(0.1)	(0.4)	(0.4)
Earnings from equity method investments	0	0	0	0
Miscellaneous income	(3.0)	(3.9)	(10.1)	(12.1)
Interest expense	(3.0)	(3.9)	(10.1)	(12.1)
Provision (benefit) for income taxes	0	0	0	0
Net income (loss) from continuing operations	0	0	0	0
Discontinued operations, net of tax	0	0	0	0
Preferred stock dividends of subsidiary	0	0	0	0
Noncontrolling interest in subsidiaries		0	0	0
Net income attributed to common shareholders	0	0	0	0
Regulated Operations
Segment Reporting Information
External revenues	606.9	582.3	2,425.1	2,116.9
Intersegment revenues	4.3	4.5	8.7	8.1
Depreciation and amortization expense	61.3	55.5	173.1	164.7
Earnings from equity method investments	22.3	21.7	66.0	63.8
Miscellaneous income	3.2	1.0	7.4	2.1
Interest expense	21.5	20.6	63.7	62.2
Provision (benefit) for income taxes	14.2	16.3	125.3	99.0
Net income (loss) from continuing operations	35.1	47.3	207.2	187.9
Discontinued operations, net of tax	0	0	0	0
Preferred stock dividends of subsidiary	(0.7)	(0.7)	(2.3)	(2.3)
Noncontrolling interest in subsidiaries		0	0	0
Net income attributed to common shareholders	34.4	46.6	204.9	185.6
Regulated Operations | Natural Gas Utility
Segment Reporting Information
External revenues	253.0	215.5	1,412.4	1,131.3
Intersegment revenues	4.2	4.5	8.6	8.1
Depreciation and amortization expense	35.6	33.2	100.1	98.3
Earnings from equity method investments	0	0	0	0
Miscellaneous income	0.4	0.1	0.8	0.6
Interest expense	12.7	11.7	37.3	35.1
Provision (benefit) for income taxes	(19.5)	(11.5)	43.1	32.6
Net income (loss) from continuing operations	(19.5)	(13.9)	72.0	53.8
Discontinued operations, net of tax	0	0	0	0
Preferred stock dividends of subsidiary	(0.1)	(0.1)	(0.4)	(0.4)
Noncontrolling interest in subsidiaries		0	0	0
Net income attributed to common shareholders	(19.6)	(14.0)	71.6	53.4
Regulated Operations | Electric Utility
Segment Reporting Information
External revenues	353.9	366.8	1,012.7	985.6
Intersegment revenues	0.1	0	0.1	0
Depreciation and amortization expense	25.7	22.3	73.0	66.4
Earnings from equity method investments	0	0	0	0
Miscellaneous income	2.8	0.9	6.6	1.5
Interest expense	8.8	8.9	26.4	27.1
Provision (benefit) for income taxes	25.1	19.5	56.9	42.4
Net income (loss) from continuing operations	40.9	47.8	94.5	94.3
Discontinued operations, net of tax	0	0	0	0
Preferred stock dividends of subsidiary	(0.6)	(0.6)	(1.9)	(1.9)
Noncontrolling interest in subsidiaries		0	0	0
Net income attributed to common shareholders	40.3	47.2	92.6	92.4
Regulated Operations | Electric Transmission Investment
Segment Reporting Information
External revenues	0	0	0	0
Intersegment revenues	0	0	0	0
Depreciation and amortization expense	0	0	0	0
Earnings from equity method investments	22.3	21.7	66.0	63.8
Miscellaneous income	0	0	0	0
Interest expense	0	0	0	0
Provision (benefit) for income taxes	8.6	8.3	25.3	24.0
Net income (loss) from continuing operations	13.7	13.4	40.7	39.8
Discontinued operations, net of tax	0	0	0	0
Preferred stock dividends of subsidiary	0	0	0	0
Noncontrolling interest in subsidiaries		0	0	0
Net income attributed to common shareholders	13.7	13.4	40.7	39.8
Nonutility and Nonregulated Operations | Integrys Energy Services
Segment Reporting Information
External revenues	512.7	335.5	1,470.7	873.7
Intersegment revenues	0.3	0.1	0.9	0.6
Depreciation and amortization expense	2.9	2.7	8.4	7.4
Earnings from equity method investments	0.5	0.5	1.2	1.2
Miscellaneous income	6.2	0.3	8.0	0.8
Interest expense	0.5	0.5	1.5	1.5
Provision (benefit) for income taxes	6.6	16.1	11.7	23.6
Net income (loss) from continuing operations	12.3	32.2	22.5	46.0
Discontinued operations, net of tax	(0.6)	(8.0)	(1.2)	(11.0)
Preferred stock dividends of subsidiary	0	0	0	0
Noncontrolling interest in subsidiaries		0	0	0
Net income attributed to common shareholders	11.7	24.2	21.3	35.0
Nonutility and Nonregulated Operations | Holding Company and Other
Segment Reporting Information
External revenues	10.1	9.9	28.1	24.6
Intersegment revenues	0.3	0.3	1.0	1.5
Depreciation and amortization expense	5.5	4.8	14.9	15.9
Earnings from equity method investments	0.3	0	1.0	0.5
Miscellaneous income	5.7	5.7	18.0	16.4
Interest expense	14.1	12.7	35.9	38.4
Provision (benefit) for income taxes	(2.8)	(2.8)	(12.7)	(16.0)
Net income (loss) from continuing operations	(8.0)	(5.2)	(12.0)	(9.1)
Discontinued operations, net of tax	0	0	5.9	1.8
Preferred stock dividends of subsidiary	0	0	0	0
Noncontrolling interest in subsidiaries		0.1	0.1	0.1
Net income attributed to common shareholders	$ (8.0)	$ (5.1)	$ (6.0)	$ (7.2)