Quarterly report pursuant to Section 13 or 15(d)

Equity and Mezzanine Equity - Additional Information (Detail)

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Equity and Mezzanine Equity - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 12 Months Ended
Nov. 30, 2019
Mar. 31, 2021
Dec. 31, 2020
Feb. 20, 2020
Feb. 19, 2020
Jan. 29, 2020
Jan. 11, 2019
Class of Stock [Line Items]              
Common stock, shares outstanding (in shares)   36,224,499 35,331,970 60,290,000 86,290,000    
Units held by noncontrolling owners (in shares)   256,994 503,033        
Gladstone Commercial Limited Partnership              
Class of Stock [Line Items]              
Company ownership percentage of voting securities   99.30% 98.60%        
Number of units redeemed in acquisition (in shares) 246,039            
Series D Preferred Stock              
Class of Stock [Line Items]              
Temporary equity, contract terms, minimum vote needed to trigger change in control from tender offer (as percent)   90.00%          
Preferred Stock | Series D Preferred Stock              
Class of Stock [Line Items]              
Redeemable preferred stock, dividend rate percentage (as percent)   7.00%          
Preferred Stock | Series E Preferred Stock              
Class of Stock [Line Items]              
Redeemable preferred stock, dividend rate percentage (as percent)   6.625%          
Preferred Stock | Series F Preferred Stock              
Class of Stock [Line Items]              
Redeemable preferred stock, shares issued (in shares)   1,500          
Consideration received   $ 30,000.00          
Maximum remaining capacity to sell common stock under open market sale agreement   633,600,000          
Redeemable preferred stock, shares authorized (in shares)       26,000,000      
Baird, Goldman Sachs, Stifel, Fifth Third, and U.S. Bancorp Investments, Inc. | Preferred Stock | Series E Preferred Stock              
Class of Stock [Line Items]              
Maximum aggregate sales price of shares to be issued under open market sale agreement   100,000,000.0          
Maximum remaining capacity to sell common stock under open market sale agreement   $ 92,800,000          
Common Stock ATM Program | Baird, Goldman Sachs, Stifel, Fifth Third, and U.S. Bancorp Investments, Inc. | Common Stock              
Class of Stock [Line Items]              
Redeemable preferred stock, shares issued (in shares)   600,000          
Consideration received   $ 11,300,000          
Maximum aggregate sales price of shares to be issued under open market sale agreement   250,000,000.0          
Maximum remaining capacity to sell common stock under open market sale agreement   172,500,000          
2019 Universal Shelf              
Class of Stock [Line Items]              
Universal registration statement, amount authorized   365,800,000         $ 500,000,000.0
2020 Universal Shelf              
Class of Stock [Line Items]              
Universal registration statement, amount authorized   $ 797,100,000       $ 800,000,000.0  
2020 Universal Shelf | Preferred Stock | Series F Preferred Stock              
Class of Stock [Line Items]              
Universal registration statement, amount authorized           $ 636,500,000