SEGMENTS OF BUSINESS (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 segment	Jun. 30, 2013
Segment reporting information
Number of reportable segments				5
Revenues		$ 1,432.6	$ 1,116.0	$ 4,357.5	$ 2,794.2
Goodwill impairment loss	0	6.7	0	6.7	0
Merger transaction costs		5.9	0	5.9	0
Transaction costs related to pending sale of UPPCO		0.9	0	0.9	0
Transaction costs related to pending sale of IES retail energy business		0.8	0	0.8	0
Depreciation and amortization expense		72.9	65.5	144.2	126.4
Earnings from equity method investments		23.9	22.8	46.8	45.1
Miscellaneous income (expense)		5.0	5.5	11.0	11.2
Interest expense		38.7	28.6	77.8	57.9
Provision (benefit) for income taxes		8.2	(3.3)	98.0	106.3
Net income (loss) from continuing operations		8.1	(3.9)	161.3	178.3
Discontinued operations, net of tax		(0.1)	(0.8)	(0.2)	5.3
Preferred stock dividends of subsidiary		(0.8)	(0.8)	(1.6)	(1.6)
Noncontrolling interest in subsidiaries		0	0.1	0.1	0.1
Net income (loss) attributed to common shareholders		7.2	(5.4)	159.6	182.1
ATC
Segment reporting information
Equity method investment, ownership interest (as a percent)		34.00%		34.00%
Earnings from equity method investments		23.0	22.0	45.5	43.7
Intersegment revenues
Segment reporting information
Revenues		0	0	0	0
Natural Gas Utility
Segment reporting information
Goodwill impairment loss				0
Integrys Energy Services
Segment reporting information
Goodwill impairment loss		6.7		6.7
Holding Company and Other
Segment reporting information
Goodwill impairment loss				0
Discontinued operations, net of tax			(0.1)		5.9
Reconciling Eliminations
Segment reporting information
Revenues		0	0	0	0
Goodwill impairment loss		0		0
Merger transaction costs		0		0
Transaction costs related to pending sale of UPPCO		0		0
Transaction costs related to pending sale of IES retail energy business		0		0
Depreciation and amortization expense		(0.2)	(0.2)	(0.3)	(0.3)
Earnings from equity method investments		0	0	0	0
Miscellaneous income (expense)		(3.2)	(3.4)	(6.7)	(7.1)
Interest expense		(3.2)	(3.4)	(6.7)	(7.1)
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 (loss) attributed to common shareholders		0	0	0	0
Reconciling Eliminations | Intersegment revenues
Segment reporting information
Revenues		(3.5)	(3.1)	(11.0)	(5.7)
Regulated operations
Segment reporting information
Revenues		806.1	694.4	2,422.8	1,818.2
Goodwill impairment loss		0		0
Merger transaction costs		0		0
Transaction costs related to pending sale of UPPCO		0		0
Transaction costs related to pending sale of IES retail energy business		0		0
Depreciation and amortization expense		63.2	58.1	125.2	111.8
Earnings from equity method investments		23.0	22.0	45.5	43.7
Miscellaneous income (expense)		2.4	2.4	6.2	4.2
Interest expense		25.3	20.4	50.4	42.2
Provision (benefit) for income taxes		12.9	23.4	106.5	111.1
Net income (loss) from continuing operations		21.1	39.6	165.8	172.1
Discontinued operations, net of tax		0	0	0	0
Preferred stock dividends of subsidiary		(0.8)	(0.8)	(1.6)	(1.6)
Noncontrolling interest in subsidiaries			0	0	0
Net income (loss) attributed to common shareholders		20.3	38.8	164.2	170.5
Regulated operations | Intersegment revenues
Segment reporting information
Revenues		2.8	2.5	7.3	4.4
Regulated operations | Natural Gas Utility
Segment reporting information
Revenues		493.5	367.4	1,761.0	1,159.4
Goodwill impairment loss		0		0
Merger transaction costs		0		0
Transaction costs related to pending sale of UPPCO		0		0
Transaction costs related to pending sale of IES retail energy business		0		0
Depreciation and amortization expense		36.9	32.3	73.3	64.5
Earnings from equity method investments		0	0	0	0
Miscellaneous income (expense)		(0.4)	0.2	(0.1)	0.4
Interest expense		13.2	11.9	26.6	24.6
Provision (benefit) for income taxes		(7.3)	(0.7)	59.4	62.6
Net income (loss) from continuing operations		(10.5)	1.7	88.7	91.5
Discontinued operations, net of tax		0	0	0	0
Preferred stock dividends of subsidiary		(0.1)	(0.2)	(0.2)	(0.3)
Noncontrolling interest in subsidiaries			0	0	0
Net income (loss) attributed to common shareholders		(10.6)	1.5	88.5	91.2
Regulated operations | Natural Gas Utility | Intersegment revenues
Segment reporting information
Revenues		2.8	2.5	7.3	4.4
Regulated operations | Electric Utility
Segment reporting information
Revenues		312.6	327.0	661.8	658.8
Goodwill impairment loss		0		0
Merger transaction costs		0		0
Transaction costs related to pending sale of UPPCO		0		0
Transaction costs related to pending sale of IES retail energy business		0		0
Depreciation and amortization expense		26.3	25.8	51.9	47.3
Earnings from equity method investments		0	0	0	0
Miscellaneous income (expense)		2.8	2.2	6.3	3.8
Interest expense		12.1	8.5	23.8	17.6
Provision (benefit) for income taxes		11.0	15.7	29.1	31.8
Net income (loss) from continuing operations		17.8	24.3	49.6	53.6
Discontinued operations, net of tax		0	0	0	0
Preferred stock dividends of subsidiary		(0.7)	(0.6)	(1.4)	(1.3)
Noncontrolling interest in subsidiaries			0	0	0
Net income (loss) attributed to common shareholders		17.1	23.7	48.2	52.3
Regulated operations | Electric Utility | Intersegment revenues
Segment reporting information
Revenues		0	0	0	0
Regulated operations | Electric Transmission Investment
Segment reporting information
Revenues		0	0	0	0
Goodwill impairment loss		0		0
Merger transaction costs		0		0
Transaction costs related to pending sale of UPPCO		0		0
Transaction costs related to pending sale of IES retail energy business		0		0
Depreciation and amortization expense		0	0	0	0
Earnings from equity method investments		23.0	22.0	45.5	43.7
Miscellaneous income (expense)		0	0	0	0
Interest expense		0	0	0	0
Provision (benefit) for income taxes		9.2	8.4	18.0	16.7
Net income (loss) from continuing operations		13.8	13.6	27.5	27.0
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 (loss) attributed to common shareholders		13.8	13.6	27.5	27.0
Regulated operations | Electric Transmission Investment | Intersegment revenues
Segment reporting information
Revenues		0	0	0	0
Nonutility and nonregulated operations | Integrys Energy Services
Segment reporting information
Revenues		600.0	412.6	1,889.6	958.0
Goodwill impairment loss		6.7		6.7
Merger transaction costs		0		0
Transaction costs related to pending sale of UPPCO		0		0
Transaction costs related to pending sale of IES retail energy business		0		0
Depreciation and amortization expense		3.0	2.8	5.9	5.5
Earnings from equity method investments		0.8	0.5	0.9	0.7
Miscellaneous income (expense)		0.4	1.4	0.7	1.8
Interest expense		0.5	0.5	1.0	1.0
Provision (benefit) for income taxes		2.4	(22.2)	8.3	5.1
Net income (loss) from continuing operations		(1.5)	(41.1)	9.4	10.2
Discontinued operations, net of tax		(0.1)	(0.7)	(0.2)	(0.6)
Preferred stock dividends of subsidiary		0	0	0	0
Noncontrolling interest in subsidiaries			0	0	0
Net income (loss) attributed to common shareholders		(1.6)	(41.8)	9.2	9.6
Nonutility and nonregulated operations | Integrys Energy Services | Intersegment revenues
Segment reporting information
Revenues		0.4	0.3	3.0	0.6
Nonutility and nonregulated operations | Holding Company and Other
Segment reporting information
Revenues		26.5	9.0	45.1	18.0
Goodwill impairment loss		0		0
Depreciation and amortization expense		6.9	4.8	13.4	9.4
Earnings from equity method investments		0.1	0.3	0.4	0.7
Miscellaneous income (expense)		5.4	5.1	10.8	12.3
Interest expense		16.1	11.1	33.1	21.8
Provision (benefit) for income taxes		(7.1)	(4.5)	(16.8)	(9.9)
Net income (loss) from continuing operations		(11.5)	(2.4)	(13.9)	(4.0)
Discontinued operations, net of tax		0	(0.1)	0	5.9
Preferred stock dividends of subsidiary		0	0	0	0
Noncontrolling interest in subsidiaries			0.1	0.1	0.1
Net income (loss) attributed to common shareholders		(11.5)	(2.4)	(13.8)	2.0
Nonutility and nonregulated operations | Holding Company and Other | Intersegment revenues
Segment reporting information
Revenues		$ 0.3	$ 0.3	$ 0.7	$ 0.7