Revenue Contract Balances (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Accounts receivable, net of allowance of $67.0 and $40.2, respectively	$ 3,661.5	$ 5,209.2
Accounts receivable, billable to clients	1,914.0	1,934.1
Contract assets	58.3	63.0
Contract liabilities (deferred revenue)	571.5	$ 585.6
Revenue, Remaining Performance Obligation, Amount	$ 715.0