Schedule I - Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net income attributed to common shareholders	$ 243.9	$ 234.3	$ 235.7	$ 420.1	$ 205.0	$ 233.2	$ 231.0	$ 390.1	$ 1,134.0	$ 1,059.3	$ 1,203.7
Reconciliation to cash provided by operating activities
Equity income in subsidiaries, net of distributions									(2.9)	(18.6)	(4.8)
Deferred income taxes									167.7	300.1	274.4
Change in -
Other current liabilities									78.7	(67.6)	161.8
Other, net									20.6	290.4	(197.4)
Net cash provided by operating activities									2,345.5	2,445.5	2,078.6
Investing activities
Acquisition of Bluewater									0.0	0.0	(226.0)
Capital contributions to subsidiaries									(52.6)	(53.5)	(109.6)
Other, net									16.5	20.5	12.0
Net cash used in investing activities									(2,494.9)	(2,384.4)	(2,254.1)
Financing activities
Exercise of stock options									67.0	29.1	30.8
Purchase of common stock									(140.1)	(72.4)	(71.3)
Dividends paid on common stock									(744.5)	(697.3)	(656.5)
Issuance of long-term debt									1,895.0	1,740.0	435.0
Retirement of long-term debt									(360.1)	(953.3)	(154.5)
Other, net									(22.4)	(15.2)	(6.5)
Net cash provided by financing activities									85.6	26.4	161.4
Cash and cash equivalents at beginning of year				84.5				38.9	84.5	38.9
Cash and cash equivalents at end of year	37.5				84.5				37.5	84.5	38.9
WEC Energy Group
Operating activities
Net income attributed to common shareholders									1,134.0	1,059.3	1,203.7
Reconciliation to cash provided by operating activities
Equity income in subsidiaries, net of distributions									(475.2)	(419.4)	(686.1)
Deferred income taxes									9.1	14.4	89.5
Change in -
Accounts receivable from related parties									3.3	(2.1)	(0.1)
Prepaid taxes									(46.5)	17.5	28.4
Accounts payable to related parties									(5.2)	4.6	(0.5)
Other current liabilities									1.5	4.7	(1.4)
Other, net									7.0	5.6	0.9
Net cash provided by operating activities									628.0	684.6	634.4
Investing activities
Acquisition of Bluewater									0.0	0.0	(226.0)
Capital contributions to subsidiaries									(602.3)	(448.7)	(173.4)
Return of capital from subsidiaries									337.3	290.2	0.0
Short-term notes receivable from related parties, net									48.5	(6.9)	167.8
Issuance of long-term notes receivable to UMERC									0.0	(100.0)	(50.0)
Redemption of long-term notes receivable from UMERC									150.0	0.0	0.0
Other, net									(0.6)	6.4	4.5
Net cash used in investing activities									(67.1)	(259.0)	(277.1)
Financing activities
Exercise of stock options									67.0	29.1	30.8
Purchase of common stock									(140.1)	(72.4)	(71.3)
Dividends paid on common stock									(744.5)	(697.3)	(656.5)
Issuance of long-term debt									350.0	600.0	0.0
Retirement of long-term debt									0.0	(300.0)	0.0
Change in short-term debt									(213.7)	53.6	173.0
Short-term notes payable to related parties, net									90.4	(6.2)	169.5
Other, net									(2.3)	(3.6)	0.0
Net cash provided by financing activities									(593.2)	(396.8)	(354.5)
Net change in cash and cash equivalents									(32.3)	28.8	2.8
Cash and cash equivalents at beginning of year				$ 32.8				$ 4.0	32.8	4.0	1.2
Cash and cash equivalents at end of year	$ 0.5				$ 32.8				$ 0.5	$ 32.8	$ 4.0