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Available-for-Sale-Securities - Available-for-Sale Securities by Contractual Maturity (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Amortized Cost    
Less than one year $ 40,006  
Greater than one year 0  
Cost 40,006 $ 51,913
Estimated Fair Value    
Less than one year 39,975  
Greater than one year 0  
Total estimated fair value $ 39,975 $ 51,885