Revenue Recognition (Deferred Revenue) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue Recognition [Abstract]
Balance at beginning of period	$ 81,600,000	$ 59,900,000	$ 45,800,000
New contracts sold	60,400,000	49,600,000	35,600,000
Balance at end of period	107,100,000	81,600,000	59,900,000
Deferred Revenue, Revenue Recognized	(34,900,000)	(27,900,000)	$ (21,500,000)
Deferred Revenue, Current	37,800,000	34,300,000
Deferred Revenue, Noncurrent	$ 69,300,000	$ 47,300,000