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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 |

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