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Common Equity - Performance Units (Details) - Performance units - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance units granted		153,465	148,036	217,560
Intrinsic value of settled performance units		$ 34.5	$ 18.7	$ 9.7
Tax benefit from distribution of performance units		$ 8.4	$ 4.4	$ 2.2
Performance units outstanding		483,842
Liability recorded on balance sheet		$ 45.9
Compensation cost not yet recognized		$ 16.7
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 7 months 6 days
Subsequent event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance units granted	152,382
Intrinsic value of settled performance units	$ 27.4
Tax benefit from distribution of performance units	$ 6.7