Revenues, Significant Changes in Unbilled Revenue (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)
Significant Changes in Unbilled Revenue [Roll Forward]
Balance at beginning of period	$ 13.3
Unbilled revenue included in opening balance transferred to other accounts during period	(3.4)	[1]
Unbilled revenue recorded during period	169.3
Unbilled revenue recorded during period transferred to other accounts	(18.3)	[1]
Other changes	0.0
Balance at end of period	$ 160.9

[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.