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Disaggregation of Revenue - Summary of Disaggregation of Revenue by Transaction Model (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disaggregation of Revenue [Line Items]                      
Revenue $ 190,205 $ 151,548 $ 123,616 $ 119,445 $ 126,148 $ 110,397 $ 89,111 $ 82,349 $ 584,814 $ 408,005 $ 296,910
Open Platform Transactions                      
Disaggregation of Revenue [Line Items]                      
Revenue                 573,242 399,945 291,331
Private Platform Transactions                      
Disaggregation of Revenue [Line Items]                      
Revenue                 $ 11,572 $ 8,060 $ 5,579