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Common Equity - Performance Units (Details) - Performance units - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award
Performance units granted		152,382	153,465	148,036
Intrinsic value of settled performance units		$ 27.7	$ 34.5	$ 18.7
Tax benefit from distribution of performance units		$ 6.8	$ 8.4	$ 4.4
Performance units outstanding		449,290
Liability recorded on balance sheet		$ 21.3
Compensation cost not yet recognized		$ 10.8
Weighted-average period over which unrecognized compensation cost is expected to be recognized		2 years
Subsequent event
Share-based Compensation Arrangement by Share-based Payment Award
Performance units granted	171,492
Intrinsic value of settled performance units	$ 15.7
Tax benefit from distribution of performance units	$ 3.8