Revenues (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues	$ 355,972	$ 245,346	$ 308,530	$ 260,178	$ 291,750	$ 227,401	$ 252,613	$ 199,427	$ 294,830	$ 300,976	$ 275,667	$ 255,504	$ 1,170,026	$ 971,191	$ 1,126,977
Service Revenues [Member]
Revenues													749,515	624,417	555,843
Commission Revenue [Member]
Revenues													420,160	394,397	366,342
Fees Revenue [Member]
Revenues													329,355	230,020	189,501
Revenue from Inventory Sales [Member]
Revenues													$ 420,511	$ 346,774	$ 571,134