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Borrowings, Line of Credit Facility (Details) - Revolving Credit Promissory Note [Member]
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Line of Credit Facility [Line Items]  
Maximum borrowing capacity under line of credit $ 12
Unused commitment fee 0.30%
Remaining borrowing capacity available for general corporate purposes $ 12
Maturity date of line of credit Nov. 30, 2018
LIBOR Rate [Member]  
Line of Credit Facility [Line Items]  
Term of variable rate 1 month
Basis spread on variable rate 2.00%
Description of variable rate basis one-month LIBOR