Revenue Contract Balances (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Accounts receivable, net of allowance of $38.8 and $42.5, respectively	$ 4,047.8	$ 5,126.6
Accounts receivable, billable to clients	2,018.7	1,900.6
Contract assets	57.8	67.9
Contract liabilities (deferred revenue)	562.4	$ 533.9
Revenue, Remaining Performance Obligation, Amount	$ 759.0