Income Tax Matters - Reconciliation of Income Tax Benefit (Provision) based on Effective Income Tax Rate and Statutory Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Reconciliation between income tax provision and statutory income tax provision:
Amount of federal income tax benefit (provision) based on the statutory rate	$ 5.0	$ (8.1)	$ (16.9)
Decrease (increase) in federal valuation allowances	0.2	0.0	(0.1)
Non-deductible compensation expense	(0.5)	(1.1)	(1.7)
Non-deductible (expense) benefit	(0.2)	0.4	0.1
State income tax benefit (provision), net of federal benefit	1.0	(1.5)	(4.5)
Research and development credit	0.6	0.4	7.7
Gross decreases (increases) for tax positions from current year	0.0	0.0	(0.3)
Gross decreases (increases) for tax positions from prior years	0.0	0.4	(2.4)
Foreign income tax expense	(0.3)	(0.4)	(0.1)
Foreign undistributed earnings	(0.3)	(0.1)	(0.2)
Income tax benefit (provision)	$ 5.5	$ (10.0)	$ (18.4)