Business and Significant Accounting Policies (Details) $ / shares in Units, enterprise in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) segment country acquisition enterprise $ / shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / shares	Dec. 31, 2014 USD ($)
Business and Significant Accounting Policies [Line Items]
Number of enterprises served | enterprise		12
Number of countries in which entity operates | country		100
Number of reportable segments | segment		4
Number of businesses acquired | acquisition		2
Acquisition and integration charges		$ 158,450,000	$ 42,598,000	$ 26,175,000
Unbilled fees receivable		66,200,000	45,700,000
Stock-based compensation expense		78,943,000	46,661,000	46,149,000
(Benefit) provision for income taxes		$ (131,096,000)	$ 94,849,000	$ 96,576,000
Basic (in Dollars per share) | $ / shares		$ 0.04	$ 2.34	$ 2.09
Diluted (in Dollars per share) | $ / shares		$ 0.04	$ 2.31	$ 2.06
Excess tax benefits from share-based compensation, operating activities		$ 0	$ 0	$ 13,860,000
Excess tax benefit from share-based compensation, financing activities		0	0	13,860,000
Adoption of ASU No. 2016-09				(261,000)
Cash and cash equivalents		538,908,000	474,233,000
Cash and cash equivalents including discontinued operations		548,900,000	474,233,000	372,976,000	$ 365,302,000
Rent expense		87,900,000	38,000,000	33,800,000
Depreciation expense		63,897,000	37,172,000	33,789,000
Net capitalized development costs for internal use software		26,900,000	16,600,000
Amortization of capitalized internal software development costs		9,900,000	8,800,000	8,200,000
Amortization of intangibles		176,274,000	24,797,000	13,342,000
Goodwill, impairment loss	$ 0
Pension expense		3,500,000	3,500,000	3,500,000
Long-term debt, gross		3,323,062,000	702,500,000
Foreign currency transaction gain (loss)		(5,500,000)	(400,000)	(2,600,000)
Derivative instruments, gain (loss) recognized in income, net		7,100,000	7,900,000	8,600,000
Payments for purchases of treasury stock		41,272,000	58,961,000	509,049,000
Connecticut Economic Development Program
Business and Significant Accounting Policies [Line Items]
Long-term debt, gross		2,500,000	2,500,000
Principal amount outstanding		2,500,000
Foreign currency exchange contracts
Business and Significant Accounting Policies [Line Items]
Derivative instruments, gain (loss) recognized in income, net		$ 800,000	(300,000)	(100,000)
Customer Relationships | Minimum
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		4 years
Customer Relationships | Maximum
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		13 years
Software | Minimum
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		3 years
Software | Maximum
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		7 years
Content | Minimum
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		1 year 6 months
Content | Maximum
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		5 years
Non-Compete | Minimum
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		2 years
Non-Compete | Maximum
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		5 years
Accumulated Earnings
Business and Significant Accounting Policies [Line Items]
Adoption of ASU No. 2016-09				(261,000)
Accounting Standards Update 2016-09
Business and Significant Accounting Policies [Line Items]
Excess tax benefit from share-based compensation, financing activities			$ 13,900,000
Accounting Standards Update 2016-09 | Pro Forma
Business and Significant Accounting Policies [Line Items]
(Benefit) provision for income taxes				$ (13,900,000)
Basic (in Dollars per share) | $ / shares				$ 0.17
Diluted (in Dollars per share) | $ / shares				$ 0.16
Excess tax benefits from share-based compensation, operating activities				$ 13,900,000
Accounting Standards Update 2016-09 | Pro Forma | Accumulated Earnings
Business and Significant Accounting Policies [Line Items]
Adoption of ASU No. 2016-09				$ (300,000)