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Fair Value Measurements - Narrative (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Fair value measurements        
Transfers between levels $ 0   $ 0  
Recurring        
Fair value measurements        
Total liabilities at Fair Value 0 $ 0   $ 0
Nonrecurring        
Fair value measurements        
Total liabilities at Fair Value $ 0 $ 0   $ 0