SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 segment	Dec. 31, 2012	Dec. 31, 2011
Segment reporting information
Number of reportable segments									5
External revenues	$ 1,710.7	$ 1,129.7	$ 1,116.0	$ 1,678.2	$ 1,197.2	$ 927.7	$ 839.6	$ 1,247.9	$ 5,634.6	$ 4,212.4	$ 4,685.9
Intersegment revenues									0	0	0
Depreciation and amortization expense									266.6	250.7	247.7
Earnings from equity method investments									91.5	87.2	79.4
Miscellaneous income									29.8	9.3	5.3
Interest expense									128.2	120.2	128.2
Provision for income taxes									210.8	149.8	133.3
Net income (loss) from continuing operations	132.3	39.4	(3.9)	182.2	69.2	74.3	51.7	98.8	350.0	294.0	230.0
Discontinued operations, net of tax									4.8	(9.7)	0.5
Preferred stock dividends of subsidiary									(3.1)	(3.1)	(3.1)
Noncontrolling interest in subsidiaries									0.1	0.2	0
Net income attributed to common shareholders	131.6	38.1	(5.4)	187.5	68.0	65.7	48.8	98.9	351.8	281.4	227.4
Total assets	11,243.5				10,327.4				11,243.5	10,327.4	9,983.2
Cash expenditures for long-lived assets									1,060.8	594.3	310.1
ATC
Segment reporting information
Equity method investment, ownership interest (as a percent)	34.00%								34.00%
Earnings from equity method investments									89.1	85.3	79.1
Holding Company and Other
Segment reporting information
Discontinued operations, net of tax									5.9	1.8	(0.5)
Reconciling Eliminations
Segment reporting information
External revenues									0	0	0
Intersegment revenues									(14.2)	(12.7)	(13.4)
Depreciation and amortization expense									(0.5)	(0.5)	(0.5)
Earnings from equity method investments									0	0	0
Miscellaneous income									(13.3)	(15.9)	(17.0)
Interest expense									(13.3)	(15.9)	(17.0)
Provision for income taxes									0	0	0
Net income (loss) from continuing operations									0	0	0
Discontinued operations, net of tax									0	0	0
Preferred stock dividends of subsidiary									0	0	0
Noncontrolling interest in subsidiaries									0	0
Net income attributed to common shareholders									0	0	0
Total assets	(786.5)				(653.7)				(786.5)	(653.7)	(578.9)
Cash expenditures for long-lived assets									0	0	0
Regulated operations
Segment reporting information
External revenues									3,425.6	2,959.5	3,294.5
Intersegment revenues									11.5	9.9	10.8
Depreciation and amortization expense									234.6	220.8	214.6
Earnings from equity method investments									89.1	85.3	79.1
Miscellaneous income									11.0	3.2	3.0
Interest expense									86.6	83.2	90.2
Provision for income taxes									181.4	143.7	151.7
Net income (loss) from continuing operations									291.3	256.8	254.7
Discontinued operations, net of tax									0	0	0
Preferred stock dividends of subsidiary									(3.1)	(3.1)	(3.1)
Noncontrolling interest in subsidiaries									0	0
Net income attributed to common shareholders									288.2	253.7	251.6
Total assets	9,694.9				8,964.1				9,694.9	8,964.1	8,455.3
Cash expenditures for long-lived assets									985.0	539.0	283.4
Regulated operations | Natural Gas Utility
Segment reporting information
External revenues									2,093.6	1,662.1	1,987.2
Intersegment revenues									11.4	9.9	10.8
Depreciation and amortization expense									136.0	131.8	126.1
Earnings from equity method investments									0	0	0
Miscellaneous income									1.2	0.6	2.2
Interest expense									50.2	47.3	48.4
Provision for income taxes									78.9	61.4	61.2
Net income (loss) from continuing operations									124.0	94.0	103.9
Discontinued operations, net of tax									0	0	0
Preferred stock dividends of subsidiary									(0.6)	(0.6)	(0.6)
Noncontrolling interest in subsidiaries									0	0
Net income attributed to common shareholders									123.4	93.4	103.3
Total assets	5,672.0				5,446.2				5,672.0	5,446.2	5,033.0
Cash expenditures for long-lived assets									370.0	375.1	199.3
Regulated operations | Electric Utility
Segment reporting information
External revenues									1,332.0	1,297.4	1,307.3
Intersegment revenues									0.1	0	0
Depreciation and amortization expense									98.6	89.0	88.5
Earnings from equity method investments									0	0	0
Miscellaneous income									9.8	2.6	0.8
Interest expense									36.4	35.9	41.8
Provision for income taxes									67.3	49.4	59.2
Net income (loss) from continuing operations									113.4	110.4	103.0
Discontinued operations, net of tax									0	0	0
Preferred stock dividends of subsidiary									(2.5)	(2.5)	(2.5)
Noncontrolling interest in subsidiaries									0	0
Net income attributed to common shareholders									110.9	107.9	100.5
Total assets	3,514.4				3,041.3				3,514.4	3,041.3	2,982.9
Cash expenditures for long-lived assets									615.0	163.9	84.1
Regulated operations | Electric Transmission Investment
Segment reporting information
External revenues									0	0	0
Intersegment revenues									0	0	0
Depreciation and amortization expense									0	0	0
Earnings from equity method investments									89.1	85.3	79.1
Miscellaneous income									0	0	0
Interest expense									0	0	0
Provision for income taxes									35.2	32.9	31.3
Net income (loss) from continuing operations									53.9	52.4	47.8
Discontinued operations, net of tax									0	0	0
Preferred stock dividends of subsidiary									0	0	0
Noncontrolling interest in subsidiaries									0	0
Net income attributed to common shareholders									53.9	52.4	47.8
Total assets	508.5				476.6				508.5	476.6	439.4
Cash expenditures for long-lived assets									0	0	0
Nonutility and nonregulated operations | Integrys Energy Services
Segment reporting information
External revenues									2,166.2	1,217.6	1,372.0
Intersegment revenues									1.3	0.9	1.1
Depreciation and amortization expense									11.4	10.3	10.3
Earnings from equity method investments									1.3	1.1	(0.7)
Miscellaneous income									8.5	1.1	1.0
Interest expense									2.0	2.1	1.7
Provision for income taxes									48.0	25.8	(7.7)
Net income (loss) from continuing operations									79.4	52.6	(7.1)
Discontinued operations, net of tax									(1.1)	(11.5)	1.0
Preferred stock dividends of subsidiary									0	0	0
Noncontrolling interest in subsidiaries									0	0
Net income attributed to common shareholders									78.3	41.1	(6.1)
Total assets	989.2				749.2				989.2	749.2	891.5
Cash expenditures for long-lived assets									15.8	30.9	16.7
Nonutility and nonregulated operations | Holding Company and Other
Segment reporting information
External revenues									42.8	35.3	19.4
Intersegment revenues									1.4	1.9	1.5
Depreciation and amortization expense									21.1	20.1	23.3
Earnings from equity method investments									1.1	0.8	1.0
Miscellaneous income									23.6	20.9	18.3
Interest expense									52.9	50.8	53.3
Provision for income taxes									(18.6)	(19.7)	(10.7)
Net income (loss) from continuing operations									(20.7)	(15.4)	(17.6)
Discontinued operations, net of tax									5.9	1.8	(0.5)
Preferred stock dividends of subsidiary									0	0	0
Noncontrolling interest in subsidiaries									0.1	0.2
Net income attributed to common shareholders									(14.7)	(13.4)	(18.1)
Total assets	1,345.9				1,267.8				1,345.9	1,267.8	1,215.3
Cash expenditures for long-lived assets									$ 60.0	$ 24.4	$ 10.0