Revenue - Summary of Total Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation Of Revenue [Line Items]
Revenues	$ 111,443	$ 84,970
U.S.
Disaggregation Of Revenue [Line Items]
Revenues	104,391	78,382
Non-U.S.
Disaggregation Of Revenue [Line Items]
Revenues	7,052	6,588
Subscription and Returns, Tax Determination | U.S.
Disaggregation Of Revenue [Line Items]
Revenues	58,971	45,232
Subscription and Returns, Tax Returns and Compliance Management | U.S.
Disaggregation Of Revenue [Line Items]
Revenues	39,433	26,686
Subscription and Returns,Interest Income on Funds Held for Customers | U.S.
Disaggregation Of Revenue [Line Items]
Revenues	525	729
Subscription and Returns
Disaggregation Of Revenue [Line Items]
Revenues	105,546	78,231
Subscription and Returns | U.S.
Disaggregation Of Revenue [Line Items]
Revenues	98,929	72,647
Professional Services
Disaggregation Of Revenue [Line Items]
Revenues	5,897	6,739
Professional Services | U.S.
Disaggregation Of Revenue [Line Items]
Revenues	$ 5,462	$ 5,735