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Line of Credit (Details) - Line of Credit [Member]
$ in Millions
12 Months Ended
Dec. 31, 2017
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) 2.106%
Line of credit basis points 2.00%
Line of credit unused portion per year (in percent) 0.375%
Revolving credit facility, expiration date Jun. 15, 2018