Common Equity - Stock Options (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Options Activity
Outstanding, shares, beginning balance	626,261	698,814
Granted, shares		59,404	81,730	80,770
Exercised, shares		(183,459)
Transferred, shares		53,399
Forfeited, shares		(1,897)
Outstanding, shares, ending balance		626,261	698,814
Options - Weighted Average Exercise Price
Outstanding, Weighted-Average Exercise Price, Beginning	$ 48.98	$ 43.64
Granted, Weighted-Average Exercise Price		68.18
Exercised, Weighted-Average Exercise Price		34.50
Transferred, Weighted-Average Exercise Price		48.30
Forfeited, Weighted Average Exercise Price		63.86
Outstanding, Weighted-Average Exercise Price, Ending		$ 48.98	$ 43.64
Options - Additional Disclosures
Outstanding, Weighted-Average Remaining Contractual Life (Years)		5 years 1 month 6 days
Outstanding, Aggregate Intrinsic Value		$ 27.1
Exercisable, shares		470,982
Exercisable, Weighted-Average Exercise Price		$ 43.85
Exercisable, Weighted-Average Remaining Contractual Life (Years)		4 years 1 month 6 days
Exercisable, Aggregate Intrinsic Value		$ 22.8
Intrinsic value of options exercised		8.0	$ 12.9	$ 11.2
Tax benefit from option exercises		2.2	$ 2.7	$ 4.5
Compensation cost not yet recognized		$ 0.8
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 7 months 6 days
Estimated weighted-average fair value per stock option (in dollars per share)		$ 8.60	$ 7.26	$ 7.12
Subsequent event
Options Activity
Granted, shares	59,511
Options - Weighted Average Exercise Price
Granted, Weighted-Average Exercise Price	$ 91.49
Options - Additional Disclosures
Estimated weighted-average fair value per stock option (in dollars per share)	$ 10.82
WEC Energy Group
Options - Additional Disclosures
Cash received by WEC Energy Group from options exercised by WE employees		$ 6.3	$ 10.0	$ 7.7