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Available-for-Sale-Securities - Available-for-Sale Securities by Contractual Maturity (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Amortized Cost    
Less than one year $ 36,139  
Greater than one year 0  
Cost 36,139 $ 37,850
Estimated Fair Value    
Less than one year 36,121  
Greater than one year 0  
Total estimated fair value $ 36,121 $ 37,818