Goodwill and Intangible Assets - Schedule of Finite-lived Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross	$ 923,199	$ 923,290
Accumulated Amortization	891,422	775,810
Net	$ 31,777	$ 147,480	$ 312,242
Weighted Average Remaining Amortization Period in Years	2 years	1 year 8 months 12 days
Client relationships
Finite-Lived Intangible Assets [Line Items]
Gross	$ 701,802	$ 701,802
Accumulated Amortization	701,518	616,187
Net	$ 284	$ 85,615	211,811
Weighted Average Remaining Amortization Period in Years	1 year 4 months 24 days	1 year
Trademarks and domain names
Finite-Lived Intangible Assets [Line Items]
Gross	$ 200,300	$ 200,300
Accumulated Amortization	169,545	139,767
Net	$ 30,755	$ 60,533	96,537
Weighted Average Remaining Amortization Period in Years	2 years	2 years 7 months 6 days
Patented technologies
Finite-Lived Intangible Assets [Line Items]
Gross	$ 19,600	$ 19,600
Accumulated Amortization	19,600	19,600
Net	0	0	3,894
Covenants not to compete
Finite-Lived Intangible Assets [Line Items]
Gross	1,497	1,588
Accumulated Amortization	759	256
Net	$ 738	$ 1,332	$ 0
Weighted Average Remaining Amortization Period in Years	1 year 9 months 18 days	2 years 6 months