Summary of Significant Accounting Policies - Change in Accumulated Balance of the Component of Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of Significant Accounting Policies
Accumulated unrealized loss on available-for-sale securities at beginning of period	$ (134)	$ (17)	$ (7)
Change during the period	117	(117)	(10)
Accumulated unrealized loss on available-for-sale securities at end of period	$ (17)	$ (134)	$ (17)