SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment reporting information
Revenues	$ 4,144.2	$ 3,485.5	$ 3,012.9
Depreciation and amortization expense	287.5	263.4	247.3
Merger transaction costs	10.4	0	0
Gain on sale of UPPCO, net of transaction costs	(85.4)	0	0
Gain on abandonment of PDI's Winnebago Energy Center	(5.0)	0	0
Earnings from equity method investments	88.3	91.5	87.2
Miscellaneous income	31.0	21.9	9.0
Interest expense	154.8	127.4	118.9
Provision for income taxes	193.4	158.0	117.9
Net income (loss) from continuing operations	278.1	267.5	238.9
Discontinued operations	1.8	87.3	45.4
Preferred stock dividends of subsidiary	(3.1)	(3.1)	(3.1)
Noncontrolling interest in subsidiaries	0.1	0.1	0.2
Net income attributed to common shareholders	276.9	351.8	281.4
Total assets	11,282.0	11,243.5	10,327.4
Cash expenditures for long-lived assets	865.0	1,060.8	594.4
ATC
Segment reporting information
Equity method investment, ownership interest (as a percent)	34.00%
Earnings from equity method investments	85.7	89.1	85.3
Intersegment revenues
Segment reporting information
Revenues	0	0	0
Reconciling Eliminations
Segment reporting information
Revenues	0	0	0
Depreciation and amortization expense	(0.5)	(0.5)	(0.5)
Merger transaction costs	0
Gain on sale of UPPCO, net of transaction costs	0
Gain on abandonment of PDI's Winnebago Energy Center	0
Earnings from equity method investments	0	0	0
Miscellaneous income	(11.8)	(12.4)	(14.1)
Interest expense	(11.8)	(12.4)	(14.1)
Provision for income taxes	0	0	0
Net income (loss) from continuing operations	0	0	0
Discontinued operations	0	0	0
Preferred stock dividends of subsidiary	0	0	0
Noncontrolling interest in subsidiaries	0	0	0
Net income attributed to common shareholders	0	0	0
Total assets	(692.2)	(786.5)	(653.7)
Cash expenditures for long-lived assets	0	0	0
Reconciling Eliminations | Intersegment revenues
Segment reporting information
Revenues	(13.9)	(12.4)	(11.2)
Regulated operations
Segment reporting information
Revenues	4,034.3	3,426.1	2,960.1
Depreciation and amortization expense	252.0	234.6	220.8
Merger transaction costs	0
Gain on sale of UPPCO, net of transaction costs	(85.4)
Gain on abandonment of PDI's Winnebago Energy Center	0
Earnings from equity method investments	85.7	89.1	85.3
Miscellaneous income	13.0	11.0	3.2
Interest expense	101.8	86.6	83.2
Provision for income taxes	203.3	181.4	143.7
Net income (loss) from continuing operations	318.3	291.3	256.8
Discontinued operations	0	0	0
Preferred stock dividends of subsidiary	(3.1)	(3.1)	(3.1)
Noncontrolling interest in subsidiaries	0	0	0
Net income attributed to common shareholders	315.2	288.2	253.7
Total assets	10,336.1	9,694.9	8,964.1
Cash expenditures for long-lived assets	743.1	985.0	539.0
Regulated operations | Intersegment revenues
Segment reporting information
Revenues	12.5	11.0	9.3
Regulated operations | Natural Gas Utility
Segment reporting information
Revenues	2,748.0	2,094.1	1,662.7
Depreciation and amortization expense	149.0	136.0	131.8
Merger transaction costs	0
Gain on sale of UPPCO, net of transaction costs	0
Gain on abandonment of PDI's Winnebago Energy Center	0
Earnings from equity method investments	0	0	0
Miscellaneous income	1.9	1.2	0.6
Interest expense	54.4	50.2	47.3
Provision for income taxes	65.6	78.9	61.4
Net income (loss) from continuing operations	100.7	124.0	94.0
Discontinued operations	0	0	0
Preferred stock dividends of subsidiary	(0.5)	(0.6)	(0.6)
Noncontrolling interest in subsidiaries	0	0	0
Net income attributed to common shareholders	100.2	123.4	93.4
Total assets	6,292.5	5,672.0	5,446.2
Cash expenditures for long-lived assets	456.5	370.0	375.1
Regulated operations | Natural Gas Utility | Intersegment revenues
Segment reporting information
Revenues	12.4	10.9	9.3
Regulated operations | Electric Utility
Segment reporting information
Revenues	1,286.3	1,332.0	1,297.4
Depreciation and amortization expense	103.0	98.6	89.0
Merger transaction costs	0
Gain on sale of UPPCO, net of transaction costs	(85.4)
Gain on abandonment of PDI's Winnebago Energy Center	0
Earnings from equity method investments	0	0	0
Miscellaneous income	11.1	9.8	2.6
Interest expense	47.4	36.4	35.9
Provision for income taxes	103.3	67.3	49.4
Net income (loss) from continuing operations	166.3	113.4	110.4
Discontinued operations	0	0	0
Preferred stock dividends of subsidiary	(2.6)	(2.5)	(2.5)
Noncontrolling interest in subsidiaries	0	0	0
Net income attributed to common shareholders	163.7	110.9	107.9
Total assets	3,506.9	3,514.4	3,041.3
Cash expenditures for long-lived assets	286.6	615.0	163.9
Regulated operations | Electric Utility | Intersegment revenues
Segment reporting information
Revenues	0.1	0.1	0
Regulated operations | Electric Transmission Investment
Segment reporting information
Revenues	0	0	0
Depreciation and amortization expense	0	0	0
Merger transaction costs	0
Gain on sale of UPPCO, net of transaction costs	0
Gain on abandonment of PDI's Winnebago Energy Center	0
Earnings from equity method investments	85.7	89.1	85.3
Miscellaneous income	0	0	0
Interest expense	0	0	0
Provision for income taxes	34.4	35.2	32.9
Net income (loss) from continuing operations	51.3	53.9	52.4
Discontinued operations	0	0	0
Preferred stock dividends of subsidiary	0	0	0
Noncontrolling interest in subsidiaries	0	0	0
Net income attributed to common shareholders	51.3	53.9	52.4
Total assets	536.7	508.5	476.6
Cash expenditures for long-lived assets	0	0	0
Regulated operations | Electric Transmission Investment | Intersegment revenues
Segment reporting information
Revenues	0	0	0
Nonutility and nonregulated operations | IES
Segment reporting information
Revenues	0	0	0
Depreciation and amortization expense	0	0	0
Merger transaction costs	0
Gain on sale of UPPCO, net of transaction costs	0
Gain on abandonment of PDI's Winnebago Energy Center	0
Earnings from equity method investments	0	0	0
Miscellaneous income	0	0	0
Interest expense	0	0	0
Provision for income taxes	0	0	0
Net income (loss) from continuing operations	0	0	0
Discontinued operations	0.4	82.5	55.1
Preferred stock dividends of subsidiary	0	0	0
Noncontrolling interest in subsidiaries	0	0	0
Net income attributed to common shareholders	0.4	82.5	55.1
Total assets	0	815.4	493.7
Cash expenditures for long-lived assets	0.9	2.6	2.0
Nonutility and nonregulated operations | IES | Intersegment revenues
Segment reporting information
Revenues	0	0	0
Nonutility and nonregulated operations | Holding Company and Other
Segment reporting information
Revenues	109.9	59.4	52.8
Depreciation and amortization expense	36.0	29.3	27.0
Merger transaction costs	10.4
Gain on sale of UPPCO, net of transaction costs	0
Gain on abandonment of PDI's Winnebago Energy Center	(5.0)
Earnings from equity method investments	2.6	2.4	1.9
Miscellaneous income	29.8	23.3	19.9
Interest expense	64.8	53.2	49.8
Provision for income taxes	(9.9)	(23.4)	(25.8)
Net income (loss) from continuing operations	(40.2)	(23.8)	(17.9)
Discontinued operations	1.4	4.8	(9.7)
Preferred stock dividends of subsidiary	0	0	0
Noncontrolling interest in subsidiaries	0.1	0.1	0.2
Net income attributed to common shareholders	(38.7)	(18.9)	(27.4)
Total assets	1,638.1	1,519.7	1,523.3
Cash expenditures for long-lived assets	121.0	73.2	53.4
Nonutility and nonregulated operations | Holding Company and Other | Intersegment revenues
Segment reporting information
Revenues	1.4	1.4	1.9
Continuing operations
Segment reporting information
Number of reportable segments	4
Discontinued operations
Segment reporting information
Number of reportable segments	1
Depreciation and amortization expense	$ 2.7	$ 3.3	$ 5.3