Revenues (Revenue from the Rendering of Services) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues [Abstract]
Commissions													$ 434,672	$ 424,128	$ 405,308
Fees													175,845	142,267	110,567
Total revenues	$ 178,785	$ 141,047	$ 166,186	$ 124,499	$ 146,769	$ 128,876	$ 158,805	$ 131,945	$ 135,462	$ 109,318	$ 155,477	$ 115,618	$ 610,517	$ 566,395	$ 515,875