Revenues, Significant Changes in Deferred Revenue (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)
Significant Changes in Deferred Revenue [Roll Forward]
Balance at beginning of period	$ 291.2
Deferred revenue included in opening balance transferred to other accounts during period	(83.2)	[1]
Deferred revenue recorded during period	277.0
Deferred revenue recorded during period transferred to other accounts	(172.6)	[1]
Other changes	(1.9)
Balance at end of period	$ 310.5

[1] Unbilled revenues are transferred to accounts receivable once we have an unconditional right to consideration from the customer.  Deferred revenues are recognized as revenue upon satisfaction of our performance obligation to the customer.