Revenues, Significant Changes in Unbilled Revenue (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
Significant Changes in Unbilled Revenue
Balance at beginning of period	$ 18.8
Unbilled revenue included in opening balance transferred to other accounts during period	(4.5)	[1]
Unbilled revenue recorded during period	56.0	[2]
Unbilled revenue recorded during period transferred to other accounts	(3.8)	[1]
Other changes	0.0
Balance at end of period	$ 66.5

[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.

[2] Unbilled revenue represents revenue that has been recognized upon satisfaction of a performance obligation, but cannot be contractually invoiced (or billed) to the customer at the balance sheet date until a future period.  Deferred revenue is recorded when payment is received from a customer prior to our satisfaction of the associated performance obligation.