Segment Information - Reconciliation of Segment Gross Contribution to Net Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Costs and expenses:
Cost of services and product development	$ 387,999	$ 367,637	$ 734,644	$ 724,846
Selling, general and administrative	514,976	460,803	1,033,746	948,548
Acquisition and integration charges (credits)	(358)	19,962	2,414	79,228
Operating income	116,002	86,096	164,801	77,385
Interest expense and other, net	24,996	36,484	50,667	70,644
Gain (loss) from divested operations	0	25,460	(2,075)	25,460
(Benefit) provision for income taxes	(12,400)	28,802	(12,142)	5,518
Net income	103,406	46,270	124,201	26,683
Reportable segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment gross contribution	687,103	645,168	$ 1,309,865	1,259,983
Reportable segments | Maximum
Costs and expenses:
Percent of target bonus allocated to segments			100.00%
Corporate and other expenses
Costs and expenses:
Cost of services and product development	4,220	11,469	$ 3,183	19,928
Selling, general and administrative	514,976	460,803	1,033,746	948,548
Depreciation and amortization	52,263	66,838	105,721	134,894
Acquisition and integration charges (credits)	$ (358)	$ 19,962	$ 2,414	$ 79,228