Segment Information - Reconciliation of Segment Gross Contribution to Net Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Cost of services and product development	$ 367,637	$ 352,004	$ 724,846	$ 589,613
Costs and expenses:
Selling, general and administrative	460,803	408,226	948,548	712,470
Acquisition and integration charges	19,962	98,332	79,228	111,604
Operating income (loss)	86,096	(98,388)	77,385	(44,874)
Interest expense and other, net	36,484	44,363	70,644	49,380
Gain from divested operations	25,460	0	25,460	0
Provision (benefit) for income taxes	28,802	(50,470)	5,518	(38,406)
Net income (loss)	46,270	(92,281)	26,683	(55,848)
Reportable segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	1,001,336	843,731	1,964,901	1,468,900
Total segment gross contribution	$ 645,168	499,036	1,259,983	887,653
Reportable segments | Maximum
Costs and expenses:
Percent of target bonus allocated to segments	100.00%
Corporate and other expenses
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Corporate and other expenses	$ (559,072)	(597,424)	(1,182,598)	(932,527)
Cost of services and product development	11,469	7,309	19,928	8,366
Costs and expenses:
Selling, general and administrative	460,803	408,226	948,548	712,470
Depreciation and amortization	66,838	83,557	134,894	100,087
Acquisition and integration charges	$ 19,962	$ 98,332	$ 79,228	$ 111,604