Income Taxes - Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
Federal	$ 0.0	$ 0.0	$ 0.0
State	0.0	0.0	0.0
Foreign	1.5	0.6	0.3
Total current	1.5	0.6	0.3
Deferred:
Federal	0.0	0.0	0.0
State	0.0	0.0	0.0
Foreign	0.0	0.0	0.0
Total deferred	0.0	0.0	0.0
Total income tax expense	$ 1.5	$ 0.6	$ 0.3