Income Tax Matters - Reconciliation of Changes in Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Gross unrecognized tax benefits at beginning of period	$ 4.1	$ 1.5	$ 1.5
Gross increases for tax positions of current year	0.1	0.3	0.0
Gross increases for tax positions of prior years	0.0	2.3	0.0
Gross decreases for tax positions of prior years	(0.4)	0.0	0.0
Gross unrecognized tax benefits at end of period	$ 3.8	$ 4.1	$ 1.5