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Cash and Cash Equivalents (Tables)
 12 Months Ended
Dec. 31, 2015
Cash and Cash Equivalents  
Schedule of summary of cash and cash equivalents
​ ​ ​
December 31,
 ​
​ ​ ​
2014
 ​ ​
2015
 ​
Cash
 ​ ​ ​ $ 5,870 ​ ​ ​ ​ $ 8,487 ​ ​
Investments with original maturity of less than three months at the time of purchase
 ​ ​ ​ ​ 18,319 ​ ​ ​ ​ ​ 11,953 ​ ​
Total cash and cash equivalents
 ​ ​ ​ $ 24,189 ​ ​ ​ ​ $ 20,440 ​ ​
​