Summary of Significant Accounting Policies Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 78,312	$ 51,713	$ 74,273	$ 55,473
Cash segregated under federal or other regulations	799	2,355	2,025	3,557
Restricted cash included in Prepaid expenses and other current assets, net	425	250	100	100
Restricted cash included in Other long-term assets	550	550	700	700
Total cash, cash equivalents, and restricted cash	$ 80,086	$ 54,868	$ 77,098	$ 59,830