Business Combinations - Schedule of Estimated Fair Values of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Cash	$ 500	$ 800
Funds held for clients	1,700	6,900
Goodwill	683,083	588,213	$ 564,300
Business Acquisitions in 2020 and 2019
Business Acquisition [Line Items]
Cash	536	826
Accounts receivable, net	5,381	1,843
Funds held for clients	1,716	6,878
Operating lease right-of-use asset, net	2,746	2,789
Other assets	431	99
Identifiable intangible assets	20,703	7,725
Operating lease liability - current	(793)	(1,013)
Other current liabilities	(3,460)	(2,245)
Operating lease liability - noncurrent	(1,953)	(1,776)
Client fund obligations	(1,716)	(6,878)
Total identifiable net assets	23,591	8,248
Goodwill	97,439	24,369
Aggregate purchase price	$ 121,030	$ 32,617