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Investments - Additional Information (Details) - USD ($)
$ in Thousands
12 Months Ended 36 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2016
Investments        
Realized gains or losses from available-for-sale securities $ 0 $ 0 $ 0  
Losses from investments 0 $ 0 $ 0 $ 0
Auction rate securities, noncurrent $ 0     $ 0