Real Estate Investments and Equity Method Investments - Additional Information (Details) ft² in Thousands																															5 Months Ended	12 Months Ended
	Feb. 16, 2018 USD ($)	Jan. 29, 2018 USD ($) shares	Jan. 09, 2018 USD ($)	Aug. 10, 2017 USD ($)	Jul. 25, 2017 USD ($)	Jul. 13, 2017 USD ($) property	Jul. 11, 2017 USD ($)	Jan. 20, 2017 USD ($)	Jan. 10, 2017 shares	Jan. 04, 2017 USD ($)	Dec. 22, 2016 USD ($)	Nov. 17, 2016 USD ($) ft²	Oct. 13, 2016 USD ($) ft² shares	Sep. 15, 2016 USD ($)	Aug. 04, 2016 USD ($) ft² shares	Jul. 29, 2016 USD ($)	Jun. 20, 2016 USD ($)	Apr. 29, 2016 USD ($) ft² shares	Jan. 14, 2016 USD ($) ft² portfolio	Jan. 08, 2016 USD ($)	Jan. 07, 2016 USD ($)	Oct. 30, 2015 USD ($)	Oct. 05, 2015 USD ($) a	Sep. 01, 2015 USD ($) a building	Jul. 10, 2015 USD ($) ft² shares	Jul. 01, 2015 USD ($) ft²	May 20, 2015 USD ($)	Apr. 08, 2015 USD ($) shares	Mar. 31, 2015 USD ($)	Jan. 05, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Feb. 25, 2016 story	Sep. 01, 2015 ft²
Real Estate Properties [Line Items]
Acquisition, common units/shares issued (in shares) | shares													2,000,000
Acquisition, contract price													$ 26,200,000
Cash consideration for acquisition																																$ 30,026,000	$ 195,645,000	$ 68,445,000
Sale of office property																																12,557,000	96,670,000	79,566,000
Equity method investments																															$ 11,411,000	11,411,000	10,235,000
Construction loan outstanding																															517,272,000	517,272,000	522,180,000
22-story mixed use tower | Durham City Center II LLC
Real Estate Properties [Line Items]
Interests in equity method investments																																			37.00%
Number stories in the mixed use tower | story																																			22
Equity method investments																															11,400,000	11,400,000	10,300,000
Guarantee of construction loan																																			37.00%
Assets being used as collateral for the construction loan																																			100.00%
Construction loan outstanding																															11,200,000	11,200,000
Borrowings under construction loans																																0
Dividends from equity investment																																0	0
Income from equity method investment																																0	0
Charlotte, NC
Real Estate Properties [Line Items]
Purchase of land conjunction with joint venture project																																	5,700,000
Town Center of Virginia Beach
Real Estate Properties [Line Items]
Payments for purchase of land																													$ 1,200,000
Newport News
Real Estate Properties [Line Items]
Payments for purchase of land																							$ 100,000
Area of property acquired | a																							3.24
Wendover Village Ourparcel Phase
Real Estate Properties [Line Items]
Rental revenues																															600,000
Net Income																															200,000
Sentara Williamsburg
Real Estate Properties [Line Items]
Net proceeds after transaction costs and tax protection payments																														$ 15,200,000
Gain (loss) on disposition of property																														6,200,000
Sale of office property																														$ 15,400,000
Whetstone Apartments
Real Estate Properties [Line Items]
Net proceeds after transaction costs and tax protection payments																											$ 35,500,000
Gain (loss) on disposition of property																											7,200,000
Sale of office property																											$ 35,600,000
Oceaneering
Real Estate Properties [Line Items]
Net proceeds after transaction costs and tax protection payments																						$ 29,000,000
Gain (loss) on disposition of property																						5,000,000
Sale of office property																						$ 30,000,000
Wawa | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Greentree Shopping Center
Real Estate Properties [Line Items]
Net proceeds after transaction costs and tax protection payments								$ 4,400,000
Gain (loss) on disposition of property								$ 3,400,000
Chesapeake And Virginia Beach, Virginia | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Real Estate Properties [Line Items]
Number of properties | property						2
Net proceeds after transaction costs and tax protection payments						$ 7,900,000
Gain (loss) on disposition of property						$ 4,200,000
Sandbridge Commons | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Real Estate Properties [Line Items]
Gain (loss) on disposition of property				$ 500,000
Proceeds from sale of land				$ 300,000
Economic Development Authority Building | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Real Estate Properties [Line Items]
Net proceeds after transaction costs and tax protection payments																					$ 6,600,000
Gain (loss) on disposition of property																					$ 400,000
Richmond Tower | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Real Estate Properties [Line Items]
Net proceeds after transaction costs and tax protection payments																				$ 77,000,000
Gain (loss) on disposition of property																				26,200,000
Sale of office property																				$ 78,000,000
Willowbrook Commons | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Real Estate Properties [Line Items]
Gain (loss) on disposition of property																	$ 100,000
Sale of office property																	$ 9,200,000
Kroger Junction | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Real Estate Properties [Line Items]
Gain (loss) on disposition of property																$ (100,000)
Sale of office property																$ 3,700,000
Oyster Point | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Real Estate Properties [Line Items]
Net proceeds after transaction costs and tax protection payments														$ 0
Gain (loss) on disposition of property														3,800,000
Sale of office property														$ 6,400,000
Brooks Crossing | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Real Estate Properties [Line Items]
Gain (loss) on disposition of property											$ 100,000
Sale of office property											$ 400,000
Operating Property Acquisitions During 2016
Real Estate Properties [Line Items]
Rental revenues																																	18,700,000
Net Income																																	$ 2,900,000
11- asset retail portfolio
Real Estate Properties [Line Items]
Number of acquired assets in portfolio | portfolio																			11
Area of property acquired | ft²																			1,100
Acquisition, total consideration																			$ 170,500,000
Southgate Square
Real Estate Properties [Line Items]
Area of property acquired | ft²																		220
Acquisition, total consideration																		$ 39,500,000
Acquisition, common units/shares issued (in shares) | shares																		1,575,185
Period for option to buy adjacent land, option one																		2 years
Future payment for option to buy adjacent land, option one																		$ 3,000,000
Future payment for option to buy adjacent land, option two																		1,250,000
Southgate Square | Class A units
Real Estate Properties [Line Items]
Acquisition, assumption of debt																		$ 21,100,000
Southshore Shops
Real Estate Properties [Line Items]
Area of property acquired | ft²															40
Acquisition, total consideration															$ 9,300,000
Acquisition, common units/shares issued (in shares) | shares															189,160
Acquisition, cash consideration															$ 6,700,000
Columbus Village II
Real Estate Properties [Line Items]
Area of property acquired | ft²													92
Acquisition, common units/shares issued (in shares) | shares													2,000,000
Acquisition, contract price													$ 26,200,000
Renaissance Place
Real Estate Properties [Line Items]
Area of property acquired | ft²												80
Renaissance Square
Real Estate Properties [Line Items]
Cash consideration for acquisition												$ 17,100,000
2015 Operating Property Acquisitions
Real Estate Properties [Line Items]
Rental revenues																																		4,800,000
Net Income																																		$ 800,000
Perry Hall Marketplace
Real Estate Properties [Line Items]
Acquisition, total consideration																												$ 35,400,000
Acquisition, common units/shares issued (in shares) | shares																												415,500
Acquisition, cash consideration																												$ 39,800,000
Socastee Commons
Real Estate Properties [Line Items]
Area of property acquired | ft²																										57
Acquisition, total consideration																										$ 8,700,000
Acquisition, assumption of debt																										5,000,000
Acquisition, cash consideration																										3,700,000
Fair value premium adjustment to the assumed debt																										$ 100,000
Columbus Village
Real Estate Properties [Line Items]
Area of property acquired | ft²																									65
Acquisition, total consideration																									$ 19,200,000
Acquisition, assumption of debt																									$ 8,800,000
Columbus Village | Class B units
Real Estate Properties [Line Items]
Acquisition, common units/shares issued (in shares) | shares																									1,000,000
Columbus Village | Class C units
Real Estate Properties [Line Items]
Acquisition, common units/shares issued (in shares) | shares									275,000																275,000
Providence Plaza
Real Estate Properties [Line Items]
Area of property acquired																								1												103
Acquisition, cash consideration																								$ 26,200,000
Number of buildings | building																								3
Greensboro, North Carolina | Wendover Village Ourparcel Phase
Real Estate Properties [Line Items]
Payments for purchase of land					$ 14,300,000
Capitalized acquisition costs					$ 100,000
Charleston, South Carolina
Real Estate Properties [Line Items]
Payments for purchase of land							$ 7,200,000			$ 7,100,000
Capitalized acquisition costs							$ 100,000			$ 200,000
Durham City Center II LLC | 22-story mixed use tower | Durham City Center II LLC
Real Estate Properties [Line Items]
Construction loan outstanding																															$ 29,200,000	$ 29,200,000
Subsequent Event | Indian Lakes Crossing
Real Estate Properties [Line Items]
Acquisition, total consideration			$ 14,700,000
Capitalized acquisition costs			$ 200,000
Subsequent Event | Parkway Centre
Real Estate Properties [Line Items]
Acquisition, total consideration		$ 11,300,000
Capitalized acquisition costs		$ 300,000
Acquisition, common units/shares issued (in shares) | shares		117,228
Acquisition, cash consideration		$ 9,600,000
Equity interests issued and issuable		1,700,000
Subsequent Event | Chesterfield, Virginia
Real Estate Properties [Line Items]
Payments for purchase of land		2,400,000
Capitalized acquisition costs		$ 100,000
Subsequent Event | Mt. Pleasant, North Carolina
Real Estate Properties [Line Items]
Payments for purchase of land	$ 2,900,000
Capitalized acquisition costs	100,000
Estimated cost of major improvements	$ 23,000,000