Revenue Contract Balances (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Accounts receivable, net of allowance of $43.6 and $42.5, respectively	$ 4,389.5	$ 5,126.6
Accounts receivable, billable to clients	1,977.6	1,900.6
Contract assets	51.2	67.9
Contract liabilities (deferred revenue)	585.2	$ 533.9
Revenue, Remaining Performance Obligation, Amount	$ 756.0