Goodwill and Intangible Assets - Schedule of Finite-lived Intangible Assets (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2020	Jun. 30, 2021
Finite-Lived Intangible Assets [Line Items]
Gross		$ 923,199	$ 1,050,372
Accumulated Amortization		891,422	922,554
Effects of Foreign Currency Translation			3,911
Net		$ 31,777	123,907
Weighted Average Remaining Amortization Period in Years	3 years	2 years
Client relationships
Finite-Lived Intangible Assets [Line Items]
Gross		$ 701,802	803,641
Accumulated Amortization		701,518	720,886
Effects of Foreign Currency Translation			3,126
Net		$ 284	79,629
Weighted Average Remaining Amortization Period in Years	3 years 3 months 18 days	1 year 4 months 24 days
Trademarks and domain names
Finite-Lived Intangible Assets [Line Items]
Gross		$ 200,300	225,177
Accumulated Amortization		169,545	181,050
Effects of Foreign Currency Translation			785
Net		$ 30,755	43,342
Weighted Average Remaining Amortization Period in Years	2 years 4 months 24 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		$ 1,497	1,954
Accumulated Amortization		759	1,018
Effects of Foreign Currency Translation			0
Net		$ 738	$ 936
Weighted Average Remaining Amortization Period in Years	2 years	1 year 9 months 18 days