Income Tax Matters - Reconciliation of Income Tax Provision based on Effective Income Tax Rate and Statutory Tax Rate (Parenthetical) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation between income tax provision and statutory income tax provision:
(Increase) decrease in federal valuation allowances	$ 0.0	$ 0.1	$ (1.7)
State and Local Jurisdiction
Reconciliation between income tax provision and statutory income tax provision:
(Increase) decrease in federal valuation allowances	(3.5)	1.4	(2.9)
State and tax expense (benefit) before adjustments	1.3	3.8	4.5
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 3.7	$ (0.7)	$ 0.9