Annual report pursuant to Section 13 and 15(d)

Related-Party Transactions - Summary of Management Fees, Incentive Fees and Associated Credits and Administration Fees (Detail)

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Related-Party Transactions - Summary of Management Fees, Incentive Fees and Associated Credits and Administration Fees (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Related Party Transaction [Line Items]      
Management fee [1] $ 2,041,000 $ 1,542,000 $ 1,343,000
Incentive fee [1] 688,000 350,000 0
Credits from voluntary, irrevocable waiver by adviser’s board of directors (54,000) 0 (321,000)
Net fees to our Adviser 2,675,000 1,892,000 1,022,000
Administration fee [1] 914,000 771,000 680,000
Financing fees to Gladstone Securities 36,000 $ 0 $ 0
Professional fees 713,000    
Taxable REIT Subsidiary      
Related Party Transaction [Line Items]      
Administration fee 22,000    
Professional fees $ 71,000    
[1] Refer to Note 6, “Related-Party Transactions,” for additional information.