Revenue Contract Balances (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Accounts receivable, net of allowance of $81.3 and $40.2, respectively	$ 3,146.6	$ 5,209.2
Accounts receivable, billable to clients	1,463.7	1,934.1
Contract assets	46.7	63.0
Contract liabilities (deferred revenue)	557.6	$ 585.6
Revenue, Remaining Performance Obligation, Amount	$ 696.0