Income Tax Matters - Reconciliation of Income Tax Provision based on Effective Income Tax Rate and Statutory Tax Rate (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:
Amount of federal income tax provision based on the statutory rate	$ (8.1)	$ (16.9)	$ (25.2)
(Increase) decrease in federal valuation allowances	0.0	(0.1)	1.7
Non-deductible compensation expense	(1.1)	(1.7)	(0.6)
Non-deductible benefit (expense)	0.4	0.1	(1.5)
State income tax provision, net of federal benefit	(1.5)	(4.5)	(2.5)
Research and development credit	0.4	7.7	0.0
Gross increases for tax positions from current year	0.0	(0.3)	0.0
Gross decreases (increases) for tax positions from prior years	0.4	(2.4)	0.0
Foreign income tax expense	(0.4)	(0.1)	(0.5)
Foreign undistributed (earnings) loss	(0.1)	(0.2)	0.4
Tax rate change	0.0	0.0	(0.1)
Income tax provision	$ 10.0	$ 18.4	$ 28.3