Amount before accumulated depreciation of building structures held for productive use including addition, improvement, or renovation to the structure, including, but not limited to, interior masonry, interior flooring, electrical, and plumbing.

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Real Estate Acquired - Allocation of Purchase Prices to Acquired Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate Properties [Line Items]
Land	$ 488,918
Buildings	1,273,275
Total Purchase Price	1,948,165	$ 1,803,200	$ 1,700,813	$ 1,622,710
Ashbrook Marketplace
Real Estate Properties [Line Items]
Land	8,776
Buildings	0
Total Purchase Price	8,776
Ashbrook Marketplace \| In-place Leases
Real Estate Properties [Line Items]
Intangible assets	0
Ashbrook Marketplace \| Above Market Rent
Real Estate Properties [Line Items]
Intangible assets	0
Ashbrook Marketplace \| Below Market Rent
Real Estate Properties [Line Items]
Intangible assets	0
7316 Wisconsin Avenue
Real Estate Properties [Line Items]
Land	38,662
Buildings	979
Total Purchase Price	40,674
7316 Wisconsin Avenue \| In-place Leases
Real Estate Properties [Line Items]
Intangible assets	(886)
7316 Wisconsin Avenue \| Above Market Rent
Real Estate Properties [Line Items]
Intangible assets	(168)
7316 Wisconsin Avenue \| Below Market Rent
Real Estate Properties [Line Items]
Intangible assets	(21)
2018 Acquisitions
Real Estate Properties [Line Items]
Land	47,438
Buildings	979
Total Purchase Price	49,450
2018 Acquisitions \| In-place Leases
Real Estate Properties [Line Items]
Intangible assets	(886)
2018 Acquisitions \| Above Market Rent
Real Estate Properties [Line Items]
Intangible assets	(168)
2018 Acquisitions \| Below Market Rent
Real Estate Properties [Line Items]
Intangible assets	$ (21)
700 N. Glebe Road
Real Estate Properties [Line Items]
Land			7,236
Buildings			0
Total Purchase Price			7,236
700 N. Glebe Road \| In-place Leases
Real Estate Properties [Line Items]
Intangible assets			0
700 N. Glebe Road \| Above Market Rent
Real Estate Properties [Line Items]
Intangible assets			0
700 N. Glebe Road \| Below Market Rent
Real Estate Properties [Line Items]
Intangible assets			0
Thruway Pad
Real Estate Properties [Line Items]
Land			2,196
Buildings			874
Total Purchase Price			3,100
Thruway Pad \| In-place Leases
Real Estate Properties [Line Items]
Intangible assets			(93)
Thruway Pad \| Above Market Rent
Real Estate Properties [Line Items]
Intangible assets			0
Thruway Pad \| Below Market Rent
Real Estate Properties [Line Items]
Intangible assets			(63)
2016 Acquisitions
Real Estate Properties [Line Items]
Land			9,432
Buildings			874
Total Purchase Price			10,336
2016 Acquisitions \| In-place Leases
Real Estate Properties [Line Items]
Intangible assets			(93)
2016 Acquisitions \| Above Market Rent
Real Estate Properties [Line Items]
Intangible assets			0
2016 Acquisitions \| Below Market Rent
Real Estate Properties [Line Items]
Intangible assets			$ (63)