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Investments - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Investments      
Realized gains or losses from available-for-sale securities $ 0 $ 0  
Losses from investments 0 $ 0  
Auction rate securities, noncurrent $ 0   $ 0