Other Intangible Assets, Net (Schedule of Finite-Lived Intangible Assets by Major Class) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets	$ 16,300	$ 14,900	$ 47,800	$ 42,900
Gross	887,589		887,589		$ 843,901
Accumulated Amortization	(412,552)		(412,552)		(368,288)
Net	475,037		475,037		475,613
Client lists
Finite-Lived Intangible Assets [Line Items]
Gross	720,525		720,525		690,693
Accumulated Amortization	(336,422)		(336,422)		(303,632)
Net	384,103		384,103		387,061
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross	33,833		33,833		28,118
Accumulated Amortization	(18,913)		(18,913)		(15,528)
Net	14,920		14,920		12,590
Trade names and licenses
Finite-Lived Intangible Assets [Line Items]
Gross	133,231		133,231		125,090
Accumulated Amortization	(57,217)		(57,217)		(49,128)
Net	$ 76,014		$ 76,014		$ 75,962