Revenue Recognition (Narrative) (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred Revenue Arrangement [Line Items]
Deferred Revenue	$ 107,100,000	$ 81,600,000	$ 59,900,000	$ 45,800,000
Deferred Revenue, Current	37,800,000	34,300,000
Deferred Revenue, Noncurrent	$ 69,300,000	$ 47,300,000