Goodwill and Intangible Assets - Schedule of Finite-lived Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Mar. 31, 2021
Finite-Lived Intangible Assets [Line Items]
Gross		$ 923,199	$ 1,049,939
Accumulated Amortization		891,422	901,314
Effects of Foreign Currency Translation			2,979
Net		$ 31,777	145,647
Weighted Average Remaining Amortization Period in Years	3 years 2 months 12 days	2 years
Client relationships
Finite-Lived Intangible Assets [Line Items]
Gross		$ 701,802	803,641
Accumulated Amortization		701,518	706,388
Effects of Foreign Currency Translation			2,211
Net		$ 284	95,042
Weighted Average Remaining Amortization Period in Years	3 years 6 months	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	174,457
Effects of Foreign Currency Translation			768
Net		$ 30,755	49,953
Weighted Average Remaining Amortization Period in Years	2 years 7 months 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		$ 1,497	1,521
Accumulated Amortization		759	869
Effects of Foreign Currency Translation			0
Net		$ 738	$ 652
Weighted Average Remaining Amortization Period in Years	1 year 8 months 12 days	1 year 9 months 18 days