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Real Estate and Intangible Assets - Additional Information (Detail) $ in Thousands |  |  |  |  |  | 3 Months Ended | 9 Months Ended
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 20, 2018 USD ($) ft² | Mar. 09, 2018 USD ($) ft² | Jul. 31, 2017 USD ($) ft² | Jul. 07, 2017 USD ($) ft² | Jun. 22, 2017 USD ($) ft² | Sep. 30, 2018 USD ($) ft² property | Sep. 30, 2017 USD ($) ft² property | Sep. 30, 2018 USD ($) ft² property | Sep. 30, 2017 USD ($) ft² property
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Number of properties acquired | property |  |  |  |  |  | 2 | 5 | 2 | 5
Purchase Price |  |  |  |  |  |  |  | $ 22,800 | $ 94,421
Change in unrealized gain (loss) related to interest rate hedging instruments, net |  |  |  |  |  | $ 245 | $ (7) | $ 1,028 | $ 172
Number of leased properties | property |  |  |  |  |  |  |  | 2 | 6
Total amortization expense related to lease intangible assets |  |  |  |  |  | 4,300 | 3,900 | $ 12,900 | $ 10,900
Amortization related to below-market lease |  |  |  |  |  | 500 | 400 | 1,500 | 1,100
Above market leases |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Amortization related to above-market lease |  |  |  |  |  | 200 | 200 | 700 | 400
Building and Tenant Improvements |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Real estate depreciation expense |  |  |  |  |  | $ 7,500 | $ 6,900 | $ 22,300 | $ 19,800
Vance, Alabama |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Aggregate Square Footage Sold | ft² |  | 127,444 |  |  |  |  |  |  |
Purchase Price |  | $ 14,300 |  |  |  |  |  |  |
Lease Term |  | 9 years 9 months 18 days |  |  |  |  |  |  |
Annualized GAAP Rent |  | $ 1,100 |  |  |  |  |  |  |
Columbus, Ohio |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Stated interest rate (as percent) | 3.22% |  |  |  |  |  |  |  |
Aggregate Square Footage Sold | ft² | 157,810 |  |  |  |  |  |  |  |
Purchase Price | $ 8,500 |  |  |  |  |  |  |  |
Lease Term | 15 years 11 days |  |  |  |  |  |  |  |
Annualized GAAP Rent | $ 800 |  |  |  |  |  |  |  |
Debt issued or assumed | $ 4,700 |  |  |  |  |  |  |  |
Conshohocken, Pennsylvania |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Stated interest rate (as percent) |  |  |  |  | 1.80% |  |  |  |
Aggregate Square Footage Sold | ft² |  |  |  |  | 60,016 |  |  |  |
Purchase Price |  |  |  |  | $ 15,700 |  |  |  |
Lease Term |  |  |  |  | 8 years 6 months |  |  |  |
Annualized GAAP Rent |  |  |  |  | $ 1,700 |  |  |  |
Debt issued or assumed |  |  |  |  | $ 11,200 |  |  |  |
Philadelphia, Pennsylvania |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Stated interest rate (as percent) |  |  |  | 3.75% |  |  |  |  |
Aggregate Square Footage Sold | ft² |  |  |  | 300,000 |  |  |  |  |
Purchase Price |  |  |  | $ 27,100 |  |  |  |  |
Lease Term |  |  |  | 15 years 4 months 24 days |  |  |  |  |
Annualized GAAP Rent |  |  |  | $ 2,300 |  |  |  |  |
Debt issued or assumed |  |  |  | $ 14,900 |  |  |  |  |
Maitland, Florida |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Stated interest rate (as percent) |  |  | 3.89% |  |  |  |  |  |
Aggregate Square Footage Sold | ft² |  |  | 306,435 |  |  |  |  |  |
Purchase Price |  |  | $ 51,600 |  |  |  |  |  |
Lease Term |  |  | 8 years 7 months 6 days |  |  |  |  |  |
Annualized GAAP Rent |  |  | $ 6,800 |  |  |  |  |  |
Debt issued or assumed |  |  | $ 28,800 |  |  |  |  |  |
Series of Property Acquisitions |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Aggregate Square Footage Sold | ft² |  |  |  |  |  | 285,254 | 666,451 | 285,254 | 666,451
Purchase Price |  |  |  |  |  |  |  | $ 22,800 | $ 94,421
Lease Term |  |  |  |  |  |  |  | 11 years 8 months 12 days | 10 years 8 months 12 days
Annualized GAAP Rent |  |  |  |  |  |  |  | $ 1,851 | $ 10,776
Debt issued or assumed |  |  |  |  |  |  |  | 4,745 | 54,887
Acquisition Expenses |  |  |  |  |  |  |  | 225 | 1,171
Interest rate swaps |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Liabilities Assumed |  |  |  |  |  |  |  | 0 | 42
Change in unrealized gain (loss) related to interest rate hedging instruments, net |  |  |  |  |  | $ 138 | $ (7) | 452 | $ 172
Interest rate swaps | Asset Acquisitions |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Change in unrealized gain (loss) related to interest rate hedging instruments, net |  |  |  |  |  |  |  | $ 700 |
LIBOR | Interest rate swaps | Columbus, Ohio |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Derivative rate (as a percent) | 5.32% |  |  |  |  |  |  |  |
LIBOR | Interest rate swaps | Conshohocken, Pennsylvania |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Derivative rate (as a percent) |  |  |  |  | 3.55% |  |  |  |

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