Goodwill and Intangible Assets - Schedule of Finite-lived Intangible Assets (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross	$ 1,051,420	$ 923,199
Accumulated amortization	(943,038)	(891,422)
Effects of Foreign Currency Translation	(7,193)
Net	$ 101,189	$ 31,777
Weighted Average Remaining Amortization Period in Years	2 years 7 months 6 days	2 years
Client relationships
Finite-Lived Intangible Assets [Line Items]
Gross	$ 803,642	$ 701,802
Accumulated amortization	(734,730)	(701,518)
Effects of Foreign Currency Translation	(5,702)
Net	$ 63,210	$ 284
Weighted Average Remaining Amortization Period in Years	2 years 10 months 24 days	1 year 4 months 24 days
Trademarks and domain names
Finite-Lived Intangible Assets [Line Items]
Gross	$ 225,177	$ 200,300
Accumulated amortization	(187,518)	(169,545)
Effects of Foreign Currency Translation	(1,491)
Net	$ 36,168	$ 30,755
Weighted Average Remaining Amortization Period in Years	2 years 1 month 6 days	2 years
Patented technologies
Finite-Lived Intangible Assets [Line Items]
Gross	$ 19,600	$ 19,600
Accumulated amortization	(19,600)	(19,600)
Effects of Foreign Currency Translation	0
Net	$ 0	$ 0
Weighted Average Remaining Amortization Period in Years	0 years	0 years
Covenants not to compete
Finite-Lived Intangible Assets [Line Items]
Gross	$ 3,001	$ 1,497
Accumulated amortization	(1,190)	(759)
Effects of Foreign Currency Translation	0
Net	$ 1,811	$ 738
Weighted Average Remaining Amortization Period in Years	2 years 4 months 24 days	1 year 9 months 18 days