Condensed Parent Company Financial Statements (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Activities
Net income	$ 546.3	$ 526.2	$ 456.5
Reconciliation to cash
Deferred income taxes, net	353.4	434.8	108.6
Accrued income taxes, net	57.9	(87.4)	(42.7)
Other current assets	12.1	(20.5)	14.1
Other current liabilities	(14.9)	44.1	22.0
Accounts receivable and accrued revenues	38.3	30.1	(50.4)
Other, net	(164.0)	(27.0)	40.0
Cash Provided by Operating Activities	1,173.9	993.4	810.4
Investing Activities
Proceeds from asset sales	8.7	41.5	68.7
Capital contributions to associated companies	(15.7)	(6.6)	(5.2)
Cash Used in Investing Activities	(729.6)	(892.5)	(633.5)
Financing Activities
Exercise of stock options	49.8	54.4	90.9
Purchase of common stock	(153.2)	(193.9)	(156.6)
Dividends paid on common stock	(276.3)	(242.0)	(187.0)
Issuance of long-term debt	251.8	720.0	530.0
Retirement and repurchase of long-term debt	(20.3)	(466.6)	(291.7)
Change in short-term debt	(275.3)	12.0	(167.2)
Other, net	0.7	4.8	9.0
Cash Used in Financing Activities	(422.8)	(111.3)	(172.6)
Change in Cash and Cash Equivalents	21.5	(10.4)	4.3
Cash and Cash Equivalents at Beginning of Year	14.1	24.5	20.2
Cash and Cash Equivalents at End of Year	35.6	14.1	24.5
Wisconsin Energy Corporation [Member]
Operating Activities
Net income	546.3	526.2	456.5
Reconciliation to cash
Equity in susidiaries' earnings	(577.6)	(545.9)	(482.9)
Dividends and distributions from subsidiaries	842.3	995.5	305.3
Deferred income taxes, net	104.4	(350.9)	(29.7)
Accrued income taxes, net	(457.9)	363.4	41.9
Other current assets	0.2	(0.1)	11.5
Other current liabilities	(6.7)	8.9	3.0
Accounts receivable and accrued revenues	22.5	(18.7)	477.2
Other, net	(8.1)	(10.2)	(1.4)
Cash Provided by Operating Activities	465.4	968.2	781.4
Investing Activities
Proceeds from asset sales	0	0	63.1
Capital contributions to associated companies	(21.5)	(36.5)	(64.5)
Capitalized interest and other	12.6	2.4	(57.3)
Cash Used in Investing Activities	(8.9)	(34.1)	(58.7)
Financing Activities
Exercise of stock options	49.8	54.4	90.9
Purchase of common stock	(153.9)	(193.9)	(156.6)
Dividends paid on common stock	(276.3)	(242.0)	(187.0)
Retirement and repurchase of long-term debt	0	(450.0)	(281.5)
Change in short-term debt	(79.5)	(116.5)	(310.5)
Change in notes payable due associated companies	3.8	3.9	106.1
Other, net	0	9.9	15.8
Cash Used in Financing Activities	(456.1)	(934.2)	(722.8)
Change in Cash and Cash Equivalents	0.4	(0.1)	(0.1)
Cash and Cash Equivalents at Beginning of Year	0.5	0.6	0.7
Cash and Cash Equivalents at End of Year	$ 0.9	$ 0.5	$ 0.6