Fair Value Measurements - Recurring Fair Value Measurements (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets
Cash equivalents - money market funds	$ 271.9
Deferred compensation plan assets	55.6	$ 59.7
Deferred purchase price receivable	132.0	41.9
Total	470.2	118.9
Liabilities
Deferred compensation plan liabilities	54.4	59.6
Earn-out liabilities	33.9	51.3
Total	89.2	114.2
Level 1
Assets
Cash equivalents - money market funds	271.9
Deferred compensation plan assets	55.6	59.7
Deferred purchase price receivable	0.0	0.0
Total	327.5	59.7
Liabilities
Deferred compensation plan liabilities	54.4	59.6
Earn-out liabilities	0.0	0.0
Total	54.4	59.6
Level 2
Assets
Cash equivalents - money market funds	0.0
Deferred compensation plan assets	0.0	0.0
Deferred purchase price receivable	0.0	0.0
Total	10.7	17.3
Liabilities
Deferred compensation plan liabilities	0.0	0.0
Earn-out liabilities	0.0	0.0
Total	0.9	3.3
Level 3
Assets
Cash equivalents - money market funds	0.0
Deferred compensation plan assets	0.0	0.0
Deferred purchase price receivable	132.0	41.9
Total	132.0	41.9
Liabilities
Deferred compensation plan liabilities	0.0	0.0
Earn-out liabilities	33.9	51.3
Total	33.9	51.3
Foreign currency forward contracts
Assets
Derivative asset	0.8	0.8
Liabilities
Derivative liability	0.9	2.2
Foreign currency forward contracts | Level 1
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Foreign currency forward contracts | Level 2
Assets
Derivative asset	0.8	0.8
Liabilities
Derivative liability	0.9	2.2
Foreign currency forward contracts | Level 3
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Cross-currency interest rate swaps
Assets
Derivative asset		7.1
Liabilities
Derivative liability		0.4
Cross-currency interest rate swaps | Level 1
Assets
Derivative asset		0.0
Liabilities
Derivative liability		0.0
Cross-currency interest rate swaps | Level 2
Assets
Derivative asset		7.1
Liabilities
Derivative liability		0.4
Cross-currency interest rate swaps | Level 3
Assets
Derivative asset		0.0
Liabilities
Derivative liability		0.0
Foreign currency net investment hedges
Liabilities
Derivative liability		0.7
Foreign currency net investment hedges | Level 1
Liabilities
Derivative liability		0.0
Foreign currency net investment hedges | Level 2
Liabilities
Derivative liability		0.7
Foreign currency net investment hedges | Level 3
Liabilities
Derivative liability		0.0
Interest rate caps
Assets
Derivative asset		8.9
Interest rate caps | Level 1
Assets
Derivative asset		0.0
Interest rate caps | Level 2
Assets
Derivative asset		8.9
Interest rate caps | Level 3
Assets
Derivative asset		0.0
Interest rate swaps
Assets
Derivative asset	9.9	0.5
Interest rate swaps | Level 1
Assets
Derivative asset	0.0	0.0
Interest rate swaps | Level 2
Assets
Derivative asset	9.9	0.5
Interest rate swaps | Level 3
Assets
Derivative asset	$ 0.0	$ 0.0