Common Equity - Restricted Shares (Details) - Restricted stock - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted Stock Activity
Outstanding, shares, beginning of period	101,087	134,109
Granted, shares		91,873
Released, shares		(122,043)
Forfeited, shares		(2,852)
Outstanding, shares, end of period		101,087	134,109
Restricted Stock Weighted-Average Grant Date Fair Value
Outstanding, weighted-average grant date fair value, beginning of period	$ 83.28	$ 66.48
Granted, weighted-average grant date fair value		91.54
Released, weighted-average grant date fair value		71.25
Forfeited, weighted-average grant date fair value		74.31
Outstanding, weighted-average grant date fair value, end of period		$ 83.28	$ 66.48
Restricted Stock - Additional Disclosures
Intrinsic value of released restricted shares		$ 11.1	$ 13.4	$ 7.9
Tax benefit from released restricted shares		3.1	$ 3.7	$ 2.2
Compensation cost not yet recognized		$ 3.1
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 9 months 18 days
Subsequent event
Restricted Stock Activity
Granted, shares	69,681
Restricted Stock Weighted-Average Grant Date Fair Value
Granted, weighted-average grant date fair value	$ 91.06