Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Non-Controlling Interest and Income Taxes		$ 122.1	$ 178.9	$ 158.6
Statutory Federal Income Tax Rate		21.00%	21.00%	21.00%
Income Taxes Computed at Statutory Federal Rate		$ 25.6	$ 37.6	$ 33.3
Increase (Decrease) in Tax Due to: [Abstract]
State Income Taxes – Net of Federal Income Tax Benefit		7.7	17.2	8.9
Production Tax Credits		(62.7)	(50.7)	(45.0)
Regulatory Differences – Excess Deferred Tax Benefit	[1]	(9.9)	(8.8)	(8.2)
U.S. Water Services Sale of Stock Basis Difference		0.0	1.7	0.0
Change in Fair Value of Contingent Consideration		0.0	0.0	(0.4)
Non-controlling interest		2.7	0.0	0.0
Other		(2.9)	(3.6)	(4.1)
Total Income Tax Expense (Benefit)		$ (39.5)	$ (6.6)	$ (15.5)
Effective Tax Rate		32.40%	3.70%	9.80%

[1] Excess deferred income taxes are being returned to customers under both the Average Rate Assumption Method and amortization periods as approved by regulators. (See Note 4. Regulatory Matters.)