Income Tax Expense (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current Income Tax Expense [Abstract]
Federal	[1]	$ 0.0	$ 0.0	$ 0.0
State	[1]	0.0	0.1	0.3
Total Current Income Tax Expense	[1]	0.0	0.1	0.3
Deferred Income Tax Expense (Benefit) [Abstract]
Federal	[2]	(48.8)	(27.8)	(26.2)
State		9.8	21.7	11.0
Investment Tax Credit Amortization		(0.5)	(0.6)	(0.6)
Total Deferred Income Tax Expense (Benefit)		(39.5)	(6.7)	(15.8)
Total Income Tax Expense (Benefit)		$ (39.5)	$ (6.6)	$ (15.5)
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	21.00%	21.00%

[1] For the years ended December 31, 2020, 2019 and 2018, the federal and state current tax expense was minimal due to NOLs which resulted from the bonus depreciation provisions of the Protecting Americans from Tax Hikes Act of 2015, the Tax Increase Prevention Act of 2014 and the American Taxpayer Relief Act of 2012. Federal and state NOLs are being carried forward to offset current and future taxable income.

[2] For the years ended December 31, 2020, 2019 and 2018, the federal tax benefit is primarily due to production tax credits.