Revenue Contract Balances (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Accounts receivable, net of allowance of $40.2 and $42.5, respectively	$ 5,209.2	$ 5,126.6
Accounts receivable, billable to clients	1,934.1	1,900.6
Contract with Customer, Asset, after Allowance for Credit Loss	63.0	67.9
Contract liabilities	585.6	$ 533.9
Revenue, remaining performance obligation, amount	$ 704.0