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Long-Term Debt - Additional Information 1 (Detail) - Revolving Credit Facility
6 Months Ended
Jun. 30, 2018
USD ($)
Debt Instrument [Line Items]  
Bank credit facility $ 50,000,000
Maturity of credit facility Mar. 04, 2019
Credit facility amount outstanding $ 0
Spread rate over LIBOR for interest rate on credit facility 2.50%
Stated percentage over federal funds rate to calculate base rate 0.50%
Stated percentage over 30 day LIBOR to calculate base rate 1.00%
Spread rate over base rate for interest rate on credit facility 1.50%
Commitment fee on unused borrowing base 0.50%
Current ratio 100.00%
Asset coverage ratio 250.00%