Income Tax Matters - Reconciliation of Income Tax (Provision) Benefit based on Effective Income Tax Rate and Statutory Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reconciliation between income tax provision and statutory income tax provision:
Amount of federal income tax (provision) benefit based on the statutory rate	$ (11.8)	$ 8.0	$ 5.0
Decrease in federal valuation allowances	0.0	1.1	0.2
Non-deductible compensation expense	(1.6)	(0.9)	(0.5)
Non-deductible benefit (expense)	0.2	(1.0)	(0.2)
State income tax benefit (provision), net of federal benefit	1.5	(0.5)	1.0
Research and development credit	3.2	2.2	0.6
Foreign income tax expense	(0.3)	(0.3)	(0.3)
Foreign undistributed earnings	(0.3)	(0.3)	(0.3)
Income tax (provision) benefit	$ (9.1)	$ 8.3	$ 5.5