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Fair Value of Assets and Liabilities - Narrative (Details)
6 Months Ended
Jun. 30, 2019
USD ($)
security
Jun. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
security
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Write down of held to maturity securities | $ $ 0 $ 0  
Fair Value, Measurements, Nonrecurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets measured at fair value, non-recurring basis | $ $ 0   $ 0
Quoted Prices in Active Markets for Identical Assets (Level 1)      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of securities | security 0   0
Significant Unobservable Inputs (Level 3)      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Number of securities | security 0   0