Real Estate Investments and Equity Method Investments - Additional Information (Details)																														3 Months Ended	12 Months Ended
	Jan. 20, 2017 USD ($)	Jan. 10, 2017 shares	Dec. 22, 2016 USD ($)	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 ($) shares	Mar. 31, 2015 USD ($)	Feb. 13, 2015 USD ($)	Jan. 05, 2015 USD ($)	Nov. 20, 2014 USD ($)	Sep. 29, 2014 USD ($)	Aug. 15, 2014 USD ($) ft² shares	May 01, 2014 USD ($)	Apr. 16, 2014 USD ($)	Jan. 17, 2014 USD ($) shares	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	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																															$ 195,645,000	$ 68,445,000	$ 2,754,000
Sale of office property																															96,670,000	79,566,000	7,387,000
Equity method investments																															10,235,000	1,411,000
Twenty Two Story Mixed Use Tower | Durham City CenterIi 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																															10,300,000
Guarantee of construction loan																																		37.00%
Assets being used as collateral for the construction loan																																		100.00%
Borrowings under construction loans																															0
Dividends from equity investment																															0
Income from equity method investment																															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]
Area of property acquired | a															3.24
Payments for purchase of land															$ 100,000
Williamsburg VA
Real Estate Properties [Line Items]
Payments for purchase of land																												$ 7,600,000
Chesapeake
Real Estate Properties [Line Items]
Payments for purchase of land																											$ 300,000
Virginia Beach, VA
Real Estate Properties [Line Items]
Payments for purchase of land																									$ 200,000
Oyster Point
Real Estate Properties [Line Items]
Sale of office property																						$ 6,500,000
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
Virginia Natural Gas
Real Estate Properties [Line Items]
Net proceeds after transaction costs and tax protection payments																								$ 7,400,000
Gain (loss) on disposition of property																								2,200,000
Sale of office property																								$ 8,900,000
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																		$ 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 Land | 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
Subsequent Event | Wawa | Disposal Group, Disposed of by Sale, Not Discontinued Operations
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
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											1,100,000
Acquisition, total consideration											$ 170,500,000
Southgate Square
Real Estate Properties [Line Items]
Area of property acquired | ft²										220,000
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							40,000
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					92,000
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				80,000
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
Stone House Square and 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																		57,000
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																	65,000
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
Columbus Village | Class C units | Subsequent Event
Real Estate Properties [Line Items]
Acquisition, common units/shares issued (in shares) | shares		275,000
Providence Plaza
Real Estate Properties [Line Items]
Area of property acquired																1																			103,000
Acquisition, cash consideration																$ 26,200,000
Number of buildings | building																3
2014 Operating Property Acquisitions
Real Estate Properties [Line Items]
Rental revenues																																	1,800,000
Net Income																																	$ (2,200,000)
Liberty Apartments
Real Estate Properties [Line Items]
Acquisition, total consideration																													$ 26,700,000
Acquisition, assumption of debt																													17,000,000
Acquisition, cash consideration																													3,000,000
Fair value discount adjustment to the assumed debt																													1,500,000
Outstanding principal balance of the assumed debt																													$ 18,500,000
Liberty Apartments | Class A units
Real Estate Properties [Line Items]
Acquisition, common units/shares issued (in shares) | shares																													695,652
Dimmock Square
Real Estate Properties [Line Items]
Area of property acquired | ft²																										106,166
Acquisition, total consideration																										$ 19,700,000
Acquisition, common units/shares issued (in shares) | shares																										990,952
Acquisition, cash consideration																										$ 10,100,000
Dimmock Square | Class A units
Real Estate Properties [Line Items]
Acquisition, common units/shares issued (in shares) | shares																										990,952