SCHEDULE I - CONDENSED PARENT COMPANY FINANCIAL STATEMENTS STATEMENTS OF INCOME (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Merger transaction costs	$ 10.4	$ 0	$ 0
Gain on sale of UPPCO, net of transaction costs	85.4	0	0
Operating income (loss)	507.0	439.5	379.5
Equity earnings from subsidiaries	88.3	91.5	87.2
Miscellaneous income	31.0	21.9	9.0
Interest expense	154.8	127.4	118.9
Other income	(35.5)	(14.0)	(22.7)
Income before taxes	471.5	425.5	356.8
Provision for income taxes	193.4	158.0	117.9
Net income from continuing operations	278.1	267.5	238.9
Net income attributed to common shareholders	276.9	351.8	281.4
Average shares of common stock
Basic (in shares)	80.2	79.5	78.6
Diluted (in shares)	80.7	80.1	79.3
Earnings per common share (basic)
Net income from continuing operations (in dollars per share)	$ 3.43	$ 3.33	$ 3.00
Discontinued operations, net of tax (in dollars per share)	$ 0.02	$ 1.10	$ 0.58
Earnings per common share (basic) (in dollars per share)	$ 3.45	$ 4.43	$ 3.58
Earnings per common share (diluted)
Net income from continuing operations (in dollars per share)	$ 3.41	$ 3.30	$ 2.98
Discontinued operations, net of tax (in dollars per share)	$ 0.02	$ 1.09	$ 0.57
Earnings per common share (diluted) (in dollars per share)	$ 3.43	$ 4.39	$ 3.55
Integrys Energy Group, Inc.
Condensed Financial Statements, Captions [Line Items]
Merger transaction costs	10.4	0	0
Operating expense	10.6	8.2	6.0
Gain on sale of UPPCO, net of transaction costs	(85.4)	0	0
Operating income (loss)	64.4	(8.2)	(6.0)
Equity earnings from subsidiaries	303.8	314.6	277.3
Miscellaneous income	23.5	18.5	21.2
Interest expense	63.8	52.1	50.0
Other income	263.5	281.0	248.5
Income before taxes	327.9	272.8	242.5
Provision for income taxes	52.8	8.3	6.5
Net income from continuing operations	275.1	264.5	236.0
Discontinued operations from Parent Company, net of tax	(18.9)	0.6	1.4
Discontinued operations from subsidiaries, net of tax	20.7	86.7	44.0
Net income attributed to common shareholders	$ 276.9	$ 351.8	$ 281.4
Average shares of common stock
Basic (in shares)	80.2	79.5	78.6
Diluted (in shares)	80.7	80.1	79.3
Earnings per common share (basic)
Net income from continuing operations (in dollars per share)	$ 3.43	$ 3.33	$ 3.00
Discontinued operations, net of tax (in dollars per share)	$ 0.02	$ 1.10	$ 0.58
Earnings per common share (basic) (in dollars per share)	$ 3.45	$ 4.43	$ 3.58
Earnings per common share (diluted)
Net income from continuing operations (in dollars per share)	$ 3.41	$ 3.30	$ 2.98
Discontinued operations, net of tax (in dollars per share)	$ 0.02	$ 1.09	$ 0.57
Earnings per common share (diluted) (in dollars per share)	$ 3.43	$ 4.39	$ 3.55