Real Estate - Additional Information (Detail)		1 Months Ended					12 Months Ended
	May 09, 2018 shares	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	May 31, 2018 USD ($) a shares	Mar. 31, 2017 USD ($)	Jan. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) property	Sep. 01, 2019 USD ($) tenant	Dec. 31, 2016 USD ($)
Real Estate Properties [Line Items]
Rent expense								$ 779,800	$ 774,700
Acquisition costs				$ 200,000
Development in process		$ 185,972,000					$ 335,644,000	185,972,000
Total purchase price		1,948,165,000					2,081,597,000	1,948,165,000	$ 1,803,200,000		$ 1,700,813,000
Number of properties acquired | property									1
Purchase price allocated to land							453,322,000
Purchase price allocated to buildings							1,292,631,000
Below market lease, carrying amount		24,800,000					24,100,000	24,800,000
Below market lease, accumulated amortization		13,100,000					13,900,000	13,100,000
Accretion income, intangible liabilities							1,500,000	1,700,000	$ 1,700,000
Lease acquisition costs
Real Estate Properties [Line Items]
Intangible assets gross carrying amount		12,500,000					11,700,000	12,500,000
Intangible assets, accumulated amortization		8,100,000					8,500,000	8,100,000
Amortization expense							$ 900,000	1,300,000	1,100,000
Weighted-average amortization period							4 years 6 months
Above Market Rent
Real Estate Properties [Line Items]
Intangible assets gross carrying amount		800,000					$ 600,000	800,000
Intangible assets, accumulated amortization		143,900					108,300	143,900
Amortization expense							$ 109,600	110,500	31,600
Weighted-average amortization period							7 years 7 months 6 days
Below Market Rent
Real Estate Properties [Line Items]
Weighted-average amortization period							5 years 3 months 18 days
Burtonsville Town Square
Real Estate Properties [Line Items]
Payments to acquire real estate						$ 76,400,000			76,400,000
Purchase price allocated to land									28,400,000
Purchase price allocated to buildings									45,800,000
Burtonsville Town Square | In-place Leases
Real Estate Properties [Line Items]
Intangible assets									2,200,000
Burtonsville Town Square | Above Market Rent
Real Estate Properties [Line Items]
Intangible assets									600,000
Burtonsville Town Square | Below Market Rent
Real Estate Properties [Line Items]
Intangible assets									$ 600,000
Olney
Real Estate Properties [Line Items]
Payments to acquire real estate					$ 3,100,000
Rent expense					$ 56,000
2018 Acquisitions
Real Estate Properties [Line Items]
Total purchase price		49,450,000						49,450,000
Purchase price allocated to land		47,438,000						47,438,000
Purchase price allocated to buildings		979,000						979,000
2018 Acquisitions | In-place Leases
Real Estate Properties [Line Items]
Intangible assets		886,000						886,000
2018 Acquisitions | Above Market Rent
Real Estate Properties [Line Items]
Intangible assets		168,000						168,000
2018 Acquisitions | Below Market Rent
Real Estate Properties [Line Items]
Intangible assets		21,000						21,000
Ashbrook Marketplace
Real Estate Properties [Line Items]
Limited partnership units issued				$ 8,800,000				8,776,000
Development in process		11,124,000					$ 19,128,000	11,124,000
7316 Wisconsin Avenue
Real Estate Properties [Line Items]
Payments to acquire real estate			$ 35,500,000
Acquisition costs			$ 700,000
Number of tenants | tenant										2
Development in process		0					$ 44,638,000	$ 0		$ 42,600,000
Parcel Adjacent to 7316 Wisconsin Avenue
Real Estate Properties [Line Items]
Payments to acquire real estate		$ 4,500,000
BF Saul Real Estate Investment Trust | Ashbrook Marketplace
Real Estate Properties [Line Items]
Limited partnership units issued (in shares) | shares	176,680			176,680
Area of land | a				13.7