Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Financial Statements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	$ 87,531	$ 73,783	$ 170,727	$ 142,259
General and administrative	17,102	14,064	34,926	29,725
Net loss	$ (11,024)	$ (2,451)	$ (21,045)	$ (7,611)
Net loss per share, basic and diluted (in dollars per share)	$ (0.09)	$ (0.02)	$ (0.18)	$ (0.07)
Net loss			$ (21,045)	$ (7,611)
Amortization of operating lease asset			1,945	2,177
Trade and client receivables			4,773	29,978
Prepaid expenses and other assets			968	585
Operating lease liability			(925)	(998)
Accrued expenses and other liabilities			7,232	(2,088)
Deferred revenue			(1,585)	(1,317)
Investing activities—decrease (increase) in restricted cash				0
Financing activities—changes in funds held in escrow, including funds in transit				0
NET CHANGE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH			49,445	(42,070)
Cash, cash equivalents, and restricted cash—beginning of period			159,603	230,067
Cash, cash equivalents, and restricted cash—end of period	$ 209,048	$ 187,997	$ 209,048	187,997
Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue		74,256		143,180
General and administrative		14,113		29,790
Net loss		(2,027)		$ (6,755)
Net loss per share, basic and diluted (in dollars per share)				$ (0.06)
Net loss				$ (6,755)
Amortization of operating lease asset				0
Trade and client receivables				30,288
Prepaid expenses and other assets				701
Operating lease liability				0
Accrued expenses and other liabilities				(430)
Deferred revenue				(408)
Investing activities—decrease (increase) in restricted cash				150
Financing activities—changes in funds held in escrow, including funds in transit				(20,116)
NET CHANGE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH				(62,036)
Cash, cash equivalents, and restricted cash—beginning of period				129,128
Cash, cash equivalents, and restricted cash—end of period		67,092		67,092
Restatement Adjustment | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue		(473)		(921)
General and administrative		0		0
Net loss		(473)		$ (921)
Net loss per share, basic and diluted (in dollars per share)				$ (0.01)
Net loss				$ (921)
Amortization of operating lease asset				0
Trade and client receivables				(310)
Prepaid expenses and other assets				0
Operating lease liability				0
Accrued expenses and other liabilities				(298)
Deferred revenue				(909)
Investing activities—decrease (increase) in restricted cash				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 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue		0		0
General and administrative		(49)		(65)
Net loss		49		$ 65
Net loss per share, basic and diluted (in dollars per share)				$ 0
Net loss				$ 65
Amortization of operating lease asset				2,177
Trade and client receivables				0
Prepaid expenses and other assets				(116)
Operating lease liability				998
Accrued expenses and other liabilities				(1,360)
Deferred revenue				0
Investing activities—decrease (increase) in restricted cash				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
Investing activities—decrease (increase) in restricted cash				(150)
Financing activities—changes in funds held in escrow, including funds in transit				20,116
NET CHANGE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH				19,966
Cash, cash equivalents, and restricted cash—beginning of period				100,939
Cash, cash equivalents, and restricted cash—end of period		$ 120,905		$ 120,905