Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Financial Statements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	$ 96,748	$ 78,015	$ 267,475	$ 220,274
General and administrative	18,047	16,468	52,973	46,193
Net loss	$ (2,747)	$ (3,547)	$ (23,792)	$ (11,158)
Net loss per share, basic and diluted (in dollars per share)	$ (0.02)	$ (0.03)	$ (0.20)	$ (0.10)
Net loss			$ (23,792)	$ (11,158)
Amortization of operating lease asset			2,927	3,059
Trade and client receivables			12,490	7,103
Prepaid expenses and other assets			284	1,407
Operating lease liability			(1,420)	(979)
Accrued expenses and other liabilities			10,448	(799)
Deferred revenue			(3,015)	(2,727)
Financing activities—changes in funds held in escrow, including funds in transit				0
NET CHANGE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH			60,104	(41,561)
Cash, cash equivalents, and restricted cash—beginning of period			159,603	230,067
Cash, cash equivalents, and restricted cash—end of period	$ 219,707	$ 188,506	$ 219,707	188,506
Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue		78,786		221,966
General and administrative		16,519		46,309
Net loss		(2,827)		$ (9,582)
Net loss per share, basic and diluted (in dollars per share)				$ (0.09)
Net loss				$ (9,582)
Amortization of operating lease asset				0
Trade and client receivables				11,885
Prepaid expenses and other assets				1,439
Operating lease liability				0
Accrued expenses and other liabilities				5,814
Deferred revenue				(1,432)
Financing activities—changes in funds held in escrow, including funds in transit				(16,407)
NET CHANGE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH				(57,968)
Cash, cash equivalents, and restricted cash—beginning of period				129,128
Cash, cash equivalents, and restricted cash—end of period		71,160		71,160
Restatement Adjustment | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue		(771)		(1,692)
General and administrative		0		0
Net loss		(771)		$ (1,692)
Net loss per share, basic and diluted (in dollars per share)				$ (0.01)
Net loss				$ (1,692)
Amortization of operating lease asset				0
Trade and client receivables				(4,782)
Prepaid expenses and other assets				0
Operating lease liability				0
Accrued expenses and other liabilities				(4,385)
Deferred revenue				(1,295)
Financing activities—changes in funds held in escrow, including funds in transit				0
NET CHANGE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH				0
Cash, cash equivalents, and restricted cash—beginning of period				0
Cash, cash equivalents, and restricted cash—end of period		0		0
Restatement Adjustment | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue		0		0
General and administrative		(51)		(116)
Net loss		51		$ 116
Net loss per share, basic and diluted (in dollars per share)				$ 0
Net loss				$ 116
Amortization of operating lease asset				3,059
Trade and client receivables				0
Prepaid expenses and other assets				(32)
Operating lease liability				(979)
Accrued expenses and other liabilities				(2,228)
Deferred revenue				0
Financing activities—changes in funds held in escrow, including funds in transit				0
NET CHANGE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH				0
Cash, cash equivalents, and restricted cash—beginning of period				0
Cash, cash equivalents, and restricted cash—end of period		0		0
Restatement Adjustment | Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue		0		0
General and administrative		0		0
Net loss		0		$ 0
Net loss per share, basic and diluted (in dollars per share)				$ 0
Net loss				$ 0
Amortization of operating lease asset				0
Trade and client receivables				0
Prepaid expenses and other assets				0
Operating lease liability				0
Accrued expenses and other liabilities				0
Deferred revenue				0
Financing activities—changes in funds held in escrow, including funds in transit				16,407
NET CHANGE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH				16,407
Cash, cash equivalents, and restricted cash—beginning of period				100,939
Cash, cash equivalents, and restricted cash—end of period		$ 117,346		$ 117,346