Common Equity - Restricted Shares (Details) - Restricted stock - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Restricted Stock Activity
Outstanding, shares, beginning of period	99,061	101,087
Granted, shares		69,681
Released, shares		(70,083)
Forfeited, shares		(1,624)
Outstanding, shares, end of period		99,061	101,087
Restricted Stock Weighted-Average Grant Date Fair Value
Outstanding, weighted-average grant date fair value, beginning of period	$ 88.89	$ 83.28
Granted, weighted-average grant date fair value		91.06
Released, weighted-average grant date fair value		83.06
Forfeited, weighted-average grant date fair value		84.43
Outstanding, weighted-average grant date fair value, end of period		$ 88.89	$ 83.28
Restricted Stock - Additional Disclosures
Intrinsic value of released restricted shares		$ 6.5	$ 11.1	$ 13.4
Tax benefit from released restricted shares		1.8	$ 3.1	$ 3.7
Compensation cost not yet recognized		$ 3.2
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 8 months 12 days
Subsequent event
Restricted Stock Activity
Granted, shares	72,211
Restricted Stock Weighted-Average Grant Date Fair Value
Granted, weighted-average grant date fair value	$ 96.04