Revenue Contract Balances (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Accounts receivable, net of allowance of $90.7 and $40.2, respectively	$ 3,297.4	$ 5,209.2
Accounts receivable, billable to clients	1,817.2	1,934.1
Contract assets	47.5	63.0
Contract liabilities (deferred revenue)	611.3	$ 585.6
Revenue, Remaining Performance Obligation, Amount	$ 667.0