Business Acquisitions - Net Assets Acquired (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Additions for current year acquisitions	$ 949,037	$ 952,404	$ 946,685
AssureEdge And Trimax
Business Acquisition [Line Items]
Cash	13,774
Consideration payable	1,827
Contingent earn-out consideration arrangement asset	0
Total consideration transferred	15,601
Recognized controlling ownership of joint venture	1,350
Business combination, equity components recorded	1,350
Business combination, total consideration transferred and equity components recorded	16,951
Cash, net of adjustment	1,358
Current assets	2,812
Property and equipment	4,021
Other assets	103
Intangible assets, definite lived	0
Capitalized software development costs	0
Deferred tax liability	0
Current and other liabilities, net of consideration transferred	(3,264)
Net assets acquired	5,030
Additions for current year acquisitions	11,921
Total net assets acquired	$ 16,951
Essel Forex, Zillious And Wallstreet Canada
Business Acquisition [Line Items]
Cash		105,391
Consideration payable		0
Contingent earn-out consideration arrangement asset		1,922
Total consideration transferred		107,313
Recognized controlling ownership of joint venture		(10,258)
Business combination, equity components recorded		(10,258)
Business combination, total consideration transferred and equity components recorded		97,055
Cash, net of adjustment		(75)
Current assets		5,175
Property and equipment		231
Other assets		3,023
Intangible assets, definite lived		6,296
Capitalized software development costs		0
Deferred tax liability		12
Current and other liabilities, net of consideration transferred		63,721
Net assets acquired		78,383
Additions for current year acquisitions		18,672
Total net assets acquired		$ 97,055