Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]
Income Before Non-Controlling Interest and Income Taxes		$ 158.6	$ 186.9	$ 175.6
Statutory Federal Income Tax Rate		21.00%	35.00%	35.00%
Income Taxes Computed at Statutory Federal Rate		$ 33.3	$ 65.4	$ 61.5
Increase (Decrease) in Tax Due to: [Abstract]
State Income Taxes – Net of Federal Income Tax Benefit		8.9	10.5	5.6
Production Tax Credits		(45.0)	(45.1)	(41.5)
Regulatory Differences – Excess Deferred Tax Benefit	[1]	(8.2)	1.2	1.4
Change in Fair Value of Contingent Consideration		(0.4)	0.0	(3.8)
Remeasurement of Deferred Income Taxes	[2]	0.0	(13.0)	0.0
Other		(4.1)	(4.3)	(3.4)
Total Income Tax Expense (Benefit)		$ (15.5)	$ 14.7	$ 19.8
Effective Tax Rate		9.80%	7.90%	11.30%

[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.)

[2] Deferred income tax benefit from the remeasurement of deferred income tax assets and liabilities resulting from the TCJA.