Revenue from Contracts with Customers - Contract Liabilities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Dec. 31, 2020
Revenue from Contracts with Customers
Contract liability, Beginning of period	$ 4.2	$ 4.2
Deferral	4.6	3.7
Revenue recognized	(4.2)	(4.0)
Contract liability, End of period	4.6	3.9
Current contract liabilities			$ 4.0	$ 3.6
Non-current contract liabilities			0.6	0.6
Total contract liabilities	$ 4.2	$ 3.9	$ 4.6	$ 4.2