Common Equity - Stock Options (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Options Activity
Outstanding, shares, beginning balance	2,887,460	3,249,918
Granted, shares		554,594	476,418	710,710
Exercised, shares		(910,083)
Forfeited, shares		(6,969)
Outstanding, shares, ending balance		2,887,460	3,249,918
Options - Weighted Average Exercise Price
Outstanding, Weighted-Average Exercise Price, Beginning	$ 64.13	$ 54.98
Granted, Weighted-Average Exercise Price		91.51
Exercised, Weighted-Average Exercise Price		48.10
Forfeited, Weighted-Average Exercise Price		71.08
Outstanding, Weighted-Average Exercise Price, Ending		$ 64.13	$ 54.98
Options - Additional Disclosures
Outstanding, Weighted-Average Remaining Contractual Life (Years)		6 years 4 months 24 days
Outstanding, Aggregate Intrinsic Value		$ 80.6
Exercisable, shares		1,461,537
Exercisable, Weighted-Average Exercise Price (in dollars per share)		$ 53.17
Exercisable, Weighted-Average Remaining Contractual Life (Years)		4 years 9 months 18 days
Exercisable, Aggregate Intrinsic Value		$ 56.8
Intrinsic value of options exercised		47.1	$ 62.4	$ 32.4
Tax benefit from option exercises		12.9	$ 17.1	$ 8.9
Compensation cost not yet recognized		$ 2.2
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 9 months 18 days
Estimated weighted-average fair value per stock option (in dollars per share)		$ 10.94	$ 8.60	$ 7.71
Subsequent event
Options Activity
Granted, shares	530,612
Options - Weighted Average Exercise Price
Granted, Weighted-Average Exercise Price	$ 91.06
Options - Additional Disclosures
Estimated weighted-average fair value per stock option (in dollars per share)	$ 13.20