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Equity and Mezzanine Equity - Additional Information (Detail) - USD ($) |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2021 | Jun. 28, 2021 | Jun. 23, 2021 | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020 | Aug. 05, 2021 | Feb. 20, 2020 | Feb. 19, 2020 | Jan. 29, 2020 | Jan. 11, 2019
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Temporary equity, contract terms, trigger change in control |  |  |  |  |  | 50.00% |  |  |  |  |  |  |
Series D preferred stock offering costs write off |  |  |  | $ 0 | $ 0 | $ 2,141,000 | $ 0 |  |  |  |  |  |
Common stock, shares authorized (in shares) |  |  |  | 62,290,000 |  | 62,290,000 |  | 60,290,000 |  |  |  |  |
Common stock, shares outstanding (in shares) |  |  |  | 36,880,119 |  | 36,880,119 |  | 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% |  | 99.30% |  | 98.60% |  |  |  |  |
Number of units redeemed in acquisition (in shares) |  |  |  |  |  | 246,039 |  |  |  |  |  |  |
Series G Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Redeemable preferred stock, dividend rate percentage |  |  | 6.00% |  |  |  |  |  |  |  |  |  |
Redeemable preferred stock, shares authorized (in shares) |  |  | 4,000,000 |  |  |  |  |  |  |  |  |  |
Series D Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Redeemable preferred stock, dividend rate percentage |  | 7.00% |  |  |  |  |  |  |  |  |  |  |
Temporary equity, contract terms, minimum vote needed to trigger change in control from tender offer (as percent) |  |  |  | 90.00% |  | 90.00% |  |  |  |  |  |  |
Shares redeemed (in shares) | 3,509,555 |  |  |  |  |  |  |  |  |  |  |  |
Redemption price (in usd per share) |  |  |  | $ 25.1458333 |  | $ 25.1458333 |  |  |  |  |  |  |
Aggregate redemption price | $ 88,300,000 |  |  |  |  |  |  |  |  |  |  |  |
Series D preferred stock offering costs write off | $ 2,100,000 |  |  |  |  |  |  |  |  |  |  |  |
Series F Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Redeemable preferred stock, shares authorized (in shares) |  |  |  | 26,000,000 |  | 26,000,000 |  | 26,000,000 |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares authorized (in shares) |  |  |  |  |  |  |  |  | 62,290,000 |  |  |  |
Common Stock | Senior Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares authorized (in shares) |  |  |  |  |  |  |  |  | 950,000 |  |  |  |
Preferred Stock | Series G Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Redeemable preferred stock, shares authorized (in shares) |  |  |  |  |  |  |  |  | 4,000,000 |  |  |  |
Preferred Stock | Series D Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Redeemable preferred stock, shares authorized (in shares) |  |  |  |  |  |  |  |  | 2,490,445 |  |  |  |
Preferred Stock | Series E Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Redeemable preferred stock, dividend rate percentage |  |  |  |  |  | 6.625% |  |  |  |  |  |  |
Redeemable preferred stock, shares authorized (in shares) |  |  |  |  |  |  |  |  | 6,760,000 |  |  |  |
Preferred Stock | Series F Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Redeemable preferred stock, shares issued (in shares) |  |  |  |  |  | 217,422 |  |  |  |  |  |  |
Consideration received |  |  |  |  |  | $ 4,900,000 |  |  |  |  |  |  |
Maximum remaining capacity to sell common stock under open market sale agreement |  |  |  | $ 628,200,000 |  | 628,200,000 |  |  |  |  |  |  |
Redeemable preferred stock, shares authorized (in shares) |  |  |  |  |  |  |  |  | 26,000,000 | 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 |  | 100,000,000 |  |  |  |  |  |  |
Maximum remaining capacity to sell common stock under open market sale agreement |  |  |  | 92,800,000 |  | $ 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) |  |  |  |  |  | 1,200,000 |  |  |  |  |  |  |
Consideration received |  |  |  |  |  | $ 24,100,000 |  |  |  |  |  |  |
Maximum aggregate sales price of shares to be issued under open market sale agreement |  |  |  | 250,000,000 |  | 250,000,000 |  |  |  |  |  |  |
Maximum remaining capacity to sell common stock under open market sale agreement |  |  |  | 159,400,000 |  | 159,400,000 |  |  |  |  |  |  |
2019 Universal Shelf |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Universal registration statement, amount authorized |  |  |  | 352,700,000 |  | 352,700,000 |  |  |  |  |  |  | $ 500,000,000
2020 Universal Shelf |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Universal registration statement, amount authorized |  |  |  | $ 691,700,000 |  | $ 691,700,000 |  |  |  |  |  | $ 800,000,000 |
2020 Universal Shelf | Preferred Stock | Series F Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Universal registration statement, amount authorized |  |  |  |  |  |  |  |  |  |  |  | $ 636,500,000 |
Over-Allotment Option | Redeemable Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Redeemable preferred stock, shares issued (in shares) |  | 4,000,000 |  |  |  |  |  |  |  |  |  |  |
Public offering price (in usd per share) |  | $ 25.00 |  |  |  |  |  |  |  |  |  |  |
Consideration received |  | $ 100,000,000 |  |  |  |  |  |  |  |  |  |  |
Gross proceeds of the offering |  | $ 96,600,000 |  |  |  |  |  |  |  |  |  |  |

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