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Organization, Basis of Presentation and Significant Accounting Policies - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2018 USD ($) Agreement subsidiary shares | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Organization And Significant Accounting Policies [Line Items] |  |  |
Number of subsidiaries interest owned in general and limited partnership | subsidiary | 2 |  |
Non-qualifying income related to real estate portfolio | $ 0 |  |
Amortization related to below-market lease | 2,000,000 | $ 1,500,000 | $ 1,200,000
Total amortization expense related to intangible assets | $ 17,700,000 | 15,900,000 | 13,400,000
Maturity period of highly-liquid investments at the time of purchase | three months or less |  |
Payments of deferred financing costs | $ 386,000 | 1,990,000 | 1,500,000
Total amortization expense related to deferred financing costs is included in interest expense | 1,445,000 | 1,713,000 | 1,932,000
Aggregate Deferred Rent Write Off | $ 200,000 | 0 | 0
Uncertain tax positions percentage | 50.00% |  |
Uncertain tax positions | $ 0 | 0 | 0
ARO liabilities accrued in connection with acquisitions | 0 | 0 | 0
Expenses recorded to general and administrative expense | 100,000 |  |
Aggregate undiscounted obligation on properties | $ 5,800,000 |  |
Number of operating ground lease arrangements | Agreement | 4 |  |
Above Market Leases |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |
Amortization related to above-market lease | $ 1,100,000 | $ 700,000 | $ 500,000
Minimum |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |
Expected lease up period for estimating carrying costs | 9 months |  |
Remaining term of in-place leases and unamortized lease | 7 years |  |
REIT taxable income to its stockholders (as percent) | 90.00% |  |
Percentage of discount rate for calculating undiscounted obligation (as percent) | 2.50% |  |
Maximum |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |
Expected lease up period for estimating carrying costs | 18 months |  |
Remaining term of in-place leases and unamortized lease | 15 years |  |
REIT taxable income to its stockholders (as percent) | 100.00% |  |
Percentage of discount rate for calculating undiscounted obligation (as percent) | 7.00% |  |
Building and Improvements |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |
Estimated useful life | 39 years |  |
Equipment and Fixtures | Minimum |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |
Estimated useful life | 5 years |  |
Equipment and Fixtures | Maximum |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |
Estimated useful life | 20 years |  |
GCLP Business Trust I |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |
Subsidiary and business trust of the Company, formed under the laws of the Commonwealth of Massachusetts, date | Dec. 28, 2005 |  |
Percentage of limited partnership interest transferred to business unit (as percent) | 99.00% |  |
Number of trust shares exchanged for limited partnership interest (in shares) | shares | 100 |  |
Operating Partnership |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |
Company ownership percentage of voting securities | 97.50% | 100.00% |
Gladstone Commercial Advisers, Inc. |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |
Company ownership percentage of voting securities | 100.00% |  |
GCLP Business Trust II |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |
Percentage of limited partnership interest transferred to business unit (as percent) | 1.00% |  |
Number of trust shares exchanged for general partnership interest (in shares) | shares | 100 |  |

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