Revenue (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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue	$ 383,413	$ 331,542	$ 389,050	$ 273,255	$ 332,194	$ 289,796	$ 393,222	$ 303,429	$ 355,972	$ 245,346	$ 308,530	$ 260,178	$ 1,377,260	$ 1,318,641	$ 1,170,026
Service Revenue [Member]
Revenue													871,596	804,024	749,515
Commission Revenue [Member]
Revenue													452,882	431,781	420,160
Fees Revenue [Member]
Revenue													418,714	372,243	329,355
Inventory Sales Revenue [Member]
Revenue													$ 505,664	$ 514,617	$ 420,511