Real Estate Acquired - Additional Information (Detail)	1 Months Ended							3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($)	Jan. 31, 2017 USD ($)	Aug. 31, 2016 USD ($)	Sep. 30, 2015 USD ($)	Feb. 28, 2015 USD ($) a	Dec. 31, 2014 USD ($)	Aug. 31, 2014 USD ($)	Dec. 31, 2016 USD ($) property	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) property	Dec. 31, 2015 USD ($) property
Real Estate Properties [Line Items]
Payments to acquire real estate									$ 17,653,000	$ 15,564,000	$ 18,855,000
Acquisition costs										60,400
Rent expense									$ 774,700	$ 843,300	$ 904,900
Number of properties acquired | property								2	1	2	1
Purchase price allocated to land									$ 450,256,000
Purchase price allocated to buildings									1,352,944,000
Below market lease, carrying amount								$ 25,100,000	25,100,000	$ 25,100,000
Below market lease, accumulated amortization								10,600,000	11,800,000	10,600,000
Accretion income, intangible liabilities									1,700,000	1,800,000	$ 1,800,000
Lease acquisition costs
Real Estate Properties [Line Items]
Intangible assets gross carrying amount								10,100,000	12,300,000	10,100,000
Intangible assets, accumulated amortization								6,400,000	7,500,000	6,400,000
Amortization expense									$ 1,100,000	1,000,000	1,300,000
Weighted-average amortization period									4 years 6 months 26 days
Above Market Rent
Real Estate Properties [Line Items]
Intangible assets gross carrying amount								10,200	$ 600,000	10,200
Intangible assets, accumulated amortization								7,800	39,500	7,800
Amortization expense									$ 31,600	1,500	1,500
Weighted-average amortization period									8 years 9 months 9 days
Below Market Rent
Real Estate Properties [Line Items]
Weighted-average amortization period									5 years 9 months 10 days
750 N. Glebe Road
Real Estate Properties [Line Items]
Payments to acquire real estate							$ 40,000,000
Acquisition costs							$ 400,000
730 N. Glebe Road
Real Estate Properties [Line Items]
Payments to acquire real estate						$ 2,800,000
Acquisition costs						$ 40,400.00
726 N Glebe Road
Real Estate Properties [Line Items]
Payments to acquire real estate				$ 4,000,000							4,000,000
Acquisition costs				$ 100,000							100,000
Purchase price allocated to land											3,900,000
Purchase price allocated to buildings											$ 100,000
700 N. Glebe Road
Real Estate Properties [Line Items]
Payments to acquire real estate			$ 7,200,000
Purchase price allocated to land								7,236,000		7,236,000
Purchase price allocated to buildings								0		0
700 N. Glebe Road | In-place Leases
Real Estate Properties [Line Items]
Leasehold Interests								0		0
700 N. Glebe Road | Above Market Rent
Real Estate Properties [Line Items]
Leasehold Interests								0		0
700 N. Glebe Road | Below Market Rent
Real Estate Properties [Line Items]
Leasehold Interests								0		0
Westview Pad
Real Estate Properties [Line Items]
Payments to acquire real estate					$ 900,000
Area of real estate property (in square feet) | a					1.1
Thruway Pad
Real Estate Properties [Line Items]
Payments to acquire real estate			3,100,000
Acquisition costs			$ 60,400
Purchase price allocated to land								2,196,000		2,196,000
Purchase price allocated to buildings								874,000		874,000
Thruway Pad | In-place Leases
Real Estate Properties [Line Items]
Leasehold Interests								93,000		93,000
Thruway Pad | Above Market Rent
Real Estate Properties [Line Items]
Leasehold Interests								0		0
Thruway Pad | Below Market Rent
Real Estate Properties [Line Items]
Leasehold Interests								63,000		63,000
Beacon Center
Real Estate Properties [Line Items]
Payments to acquire real estate								22,700,000
Rent expense										60,000
Mortgage loans on real estate, period increase (decrease)								11,250,000
Southdale
Real Estate Properties [Line Items]
Payments to acquire real estate								15,300,000
Rent expense										60,000
Burtonsville Town Square
Real Estate Properties [Line Items]
Payments to acquire real estate		$ 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]
Leasehold Interests									2,200,000
Burtonsville Town Square | Above Market Rent
Real Estate Properties [Line Items]
Leasehold Interests									600,000
Burtonsville Town Square | Below Market Rent
Real Estate Properties [Line Items]
Leasehold Interests									$ 600,000
2016 Acquisitions
Real Estate Properties [Line Items]
Payments to acquire real estate										10,300,000
Purchase price allocated to land								9,432,000		9,432,000
Purchase price allocated to buildings								874,000		874,000
2016 Acquisitions | In-place Leases
Real Estate Properties [Line Items]
Leasehold Interests								93,000		93,000
2016 Acquisitions | Above Market Rent
Real Estate Properties [Line Items]
Leasehold Interests								0		0
2016 Acquisitions | Below Market Rent
Real Estate Properties [Line Items]
Leasehold Interests								$ 63,000		$ 63,000
Olney
Real Estate Properties [Line Items]
Payments to acquire real estate	$ 3,100,000
Rent expense	$ 56,000