Business Acquisitions - Narrative (Details) - USD ($)				1 Months Ended	12 Months Ended
	May 04, 2020	Aug. 23, 2019	Jan. 01, 2019	Oct. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Oct. 01, 2020
Business Acquisition [Line Items]
Reduction of acquisition earn-out contingent liability					$ 3,105,000	$ 16,543,000	$ 1,391,000
Contingent Earn-Out Provision					0	10,095,000	24,976,000
Contingent liability for earn-out acquisition consideration					0	1,474,000
Contingent liability for earn-out acquisition consideration					0	8,621,000
Goodwill					949,037,000	952,404,000	946,685,000
Amortization expense, acquired intangible assets					9,500,000	10,200,000	$ 7,500,000
Final Allocation
Business Acquisition [Line Items]
Contingent Earn-Out Provision					0	1,922,000
Goodwill					11,921,000	17,932,000
Wallstreet Canada
Business Acquisition [Line Items]
Business acquisition, cost of acquired entity, cash paid		$ 2,100,000
Wallstreet Canada | Final Allocation
Business Acquisition [Line Items]
Contingent Earn-Out Provision		0
Goodwill		$ 71,000
Essel
Business Acquisition [Line Items]
Business acquisition, cost of acquired entity, cash paid			$ 8,700,000
Essel | Final Allocation
Business Acquisition [Line Items]
Contingent Earn-Out Provision			407,000
Goodwill			8,372,000
Zillious
Business Acquisition [Line Items]
Business acquisition, cost of acquired entity, cash paid			10,100,000
Zillious | Final Allocation
Business Acquisition [Line Items]
Contingent Earn-Out Provision			1,515,000
Goodwill			9,489,000
Trimax
Business Acquisition [Line Items]
Business acquisition, cost of acquired entity, cash paid	$ 9,900,000
Preferred interest, term	5 years
Maximum value of preferred interest	$ 9,900,000
Trimax | Final Allocation
Business Acquisition [Line Items]
Contingent Earn-Out Provision	0
Goodwill	$ 8,243,000
AsureEdge
Business Acquisition [Line Items]
Business acquisition, cost of acquired entity, cash paid				$ 5,000,000.0
Ownership percentage				70.00%
AsureEdge | Final Allocation
Business Acquisition [Line Items]
Contingent Earn-Out Provision								$ 0
Goodwill								$ 3,678,000
Maximum | Essel
Business Acquisition [Line Items]
Contingent Earn-Out Provision			721,000
Maximum | Zillious
Business Acquisition [Line Items]
Contingent Earn-Out Provision			$ 2,200,000
Zillious
Business Acquisition [Line Items]
Ownership percentage			80.00%
Contingent Accrued Earn-out Acquisition Consideration
Business Acquisition [Line Items]
Contingent Earn-Out Provision					$ 0	$ 10,100,000