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Line of Credit (Details) - Line of Credit [Member]
$ in Millions
 12 Months Ended
Dec. 31, 2018
USD ($)
Line of Credit Facility [Line Items]  
Credit facility with a bank $ 40
Description of interest rate of credit facility LIBOR
Line of credit interest rate (in percent) 2.503%
Line of credit basis points 1.50%
Line of credit unused portion per year (in percent) 0.25%
Revolving credit facility, expiration date Aug. 31, 2019