Business Acquisitions - Recognized Intangible Assets, Goodwill and Earn-Out Provisions (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Oct. 01, 2020	May 04, 2020	Dec. 31, 2019	Aug. 23, 2019	Jan. 01, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill	$ 949,037			$ 952,404			$ 946,685
Contingent Earn-Out Provision	0			10,095			$ 24,976
Final Allocation
Business Acquisition [Line Items]
Goodwill	11,921			17,932
Intangible Assets	0			3,038
Contingent Earn-Out Provision	$ 0			$ 1,922
Final Allocation | Essel
Business Acquisition [Line Items]
Goodwill						$ 8,372
Intangible Assets						1,163
Contingent Earn-Out Provision						407
Final Allocation | Zillious
Business Acquisition [Line Items]
Goodwill						9,489
Intangible Assets						1,875
Contingent Earn-Out Provision						$ 1,515
Final Allocation | Wallstreet Canada
Business Acquisition [Line Items]
Goodwill					$ 71
Intangible Assets					0
Contingent Earn-Out Provision					$ 0
Final Allocation | Trimax
Business Acquisition [Line Items]
Goodwill			$ 8,243
Intangible Assets			0
Contingent Earn-Out Provision			$ 0
Final Allocation | AsureEdge
Business Acquisition [Line Items]
Goodwill		$ 3,678
Intangible Assets		0
Contingent Earn-Out Provision		$ 0