Preferred Stock - Options Exercised (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Preferred stock
Cash proceeds from exercise of stock options	$ 2,731	$ 1,729
Series A convertible preferred stock
Preferred stock
Exercise of stock options (in shares)	360,385
Cash proceeds from exercise of stock options	$ 211

Number of share options (or share units) exercised during the current period.

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Period Type:	duration