Real Estate Properties - Summary of Properties Disposed (Details) $ in Thousands	1 Months Ended																12 Months Ended
	Dec. 31, 2018 USD ($) ft² building property	Sep. 30, 2018 USD ($) ft² building property	Aug. 31, 2018 USD ($) ft² building property	May 31, 2018 USD ($) ft² building property	Mar. 31, 2018 USD ($) ft² building property	Feb. 28, 2018 USD ($) ft² building property	Dec. 31, 2017 building	Dec. 31, 2017 building property	Dec. 31, 2017 ft² building	Dec. 31, 2017 USD ($) building	Aug. 31, 2017 USD ($) ft² building property	Jul. 31, 2017 USD ($) ft² building property	May 31, 2017 USD ($) ft² building property	Apr. 30, 2017 USD ($) ft² building property	Mar. 31, 2017 USD ($) ft² building property	Jan. 31, 2017 USD ($) ft² building property	Dec. 31, 2018 USD ($) ft² building property	Dec. 31, 2017 USD ($) ft² building property	Dec. 31, 2016 USD ($) building
Real Estate Properties [Line Items]
Gain (Loss) on Sale																	$ 251,417	$ 15,498	$ 250,886
Disposed of by Sale
Real Estate Properties [Line Items]
Number of Properties | property																	7	16
Number of Buildings | building																	9	37	62
Square Footage (in sqft) | ft²																	4,404,797	6,588,128
Gross Sales Price																	$ 1,031,286	$ 862,624	$ 1,300,000
Gain (Loss) on Sale																	251,458	15,493
Net income included in continued operations																	$ 110,600	$ 9,300	$ 12,200
Disposed of by Sale | 1600 Market Street
Real Estate Properties [Line Items]
Number of Properties | property						1
Number of Buildings | building						1
Square Footage (in sqft) | ft²						825,968
Gross Sales Price						$ 160,000
Gain (Loss) on Sale						$ 54,599
Disposed of by Sale | 600 West Chicago Avenue
Real Estate Properties [Line Items]
Number of Properties | property						1
Number of Buildings | building						2
Square Footage (in sqft) | ft²						1,561,477
Gross Sales Price						$ 510,000
Gain (Loss) on Sale						$ 107,790
Disposed of by Sale | 5073, 5075, & 5085 S. Syracuse Street
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings | building					1
Square Footage (in sqft) | ft²					248,493
Gross Sales Price					$ 115,186
Gain (Loss) on Sale					$ 42,762
Disposed of by Sale | 1601 Dry Creek Drive
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings | building				1
Square Footage (in sqft) | ft²				552,865
Gross Sales Price				$ 68,500
Gain (Loss) on Sale				$ 26,979
Disposed of by Sale | 777 East Eisenhower Parkway
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings | building			1
Square Footage (in sqft) | ft²			290,530
Gross Sales Price			$ 29,500
Gain (Loss) on Sale			$ 5,308
Disposed of by Sale | 8750 Bryn Mawr Avenue
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings | building		2
Square Footage (in sqft) | ft²		636,078
Gross Sales Price		$ 141,000
Gain (Loss) on Sale		$ 15,194
Disposed of by Sale | 97 Newberry Road
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings | building	1
Square Footage (in sqft) | ft²	289,386
Gross Sales Price	$ 7,100
Gain (Loss) on Sale	$ (1,174)
Disposed of by Sale | 111 Market Place
Real Estate Properties [Line Items]
Number of Properties | property																1
Number of Buildings | building																1
Square Footage (in sqft) | ft²																589,380
Gross Sales Price																$ 60,100
Gain (Loss) on Sale																$ (5,968)
Disposed of by Sale | Cabot Business Park Land
Real Estate Properties [Line Items]
Number of Properties | property															0
Number of Buildings | building															0
Square Footage (in sqft) | ft²															0
Gross Sales Price															$ 575
Gain (Loss) on Sale															$ (57)
Disposed of by Sale | Parkshore Plaza
Real Estate Properties [Line Items]
Number of Properties | property														1
Number of Buildings | building														4
Square Footage (in sqft) | ft²														271,072
Gross Sales Price														$ 40,000
Gain (Loss) on Sale														$ (2,460)
Disposed of by Sale | 25 S. Charles Street
Real Estate Properties [Line Items]
Number of Properties | property														1
Number of Buildings | building														1
Square Footage (in sqft) | ft²														359,254
Gross Sales Price														$ 24,500
Gain (Loss) on Sale														$ (3,487)
Disposed of by Sale | 802 Delaware Avenue
Real Estate Properties [Line Items]
Number of Properties | property													1
Number of Buildings | building													1
Square Footage (in sqft) | ft²													240,780
Gross Sales Price													$ 34,000
Gain (Loss) on Sale													$ 9,099
Disposed of by Sale | 1500 Market Street
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												1
Square Footage (in sqft) | ft²												1,759,193
Gross Sales Price												$ 328,000
Gain (Loss) on Sale												$ 38,585
Disposed of by Sale | 6600 North Military Trail
Real Estate Properties [Line Items]
Number of Properties | property											1
Number of Buildings | building											3
Square Footage (in sqft) | ft²											639,825
Gross Sales Price											$ 132,050
Gain (Loss) on Sale											$ (14,175)
Disposed of by Sale | 789 East Eisenhower Parkway
Real Estate Properties [Line Items]
Number of Properties							1	0
Number of Buildings | building							1
Square Footage (in sqft) | ft²									130,961
Gross Sales Price										$ 24,942
Gain (Loss) on Sale										$ 1,242
Number of buildings contained in property | building							2	2	2	2								2
Disposed of by Sale | 33 Stiles Lane
Real Estate Properties [Line Items]
Number of Properties | property								1
Number of Buildings | building							1
Square Footage (in sqft) | ft²									175,301
Gross Sales Price										$ 10,500
Gain (Loss) on Sale										2,163
Disposed of by Sale | 625 Crane Street (Land)
Real Estate Properties [Line Items]
Number of Properties | property								0
Number of Buildings | building							0
Square Footage (in sqft) | ft²									0
Gross Sales Price										307
Gain (Loss) on Sale										249
Disposed of by Sale | Mineral Rights
Real Estate Properties [Line Items]
Number of Properties | property								0
Number of Buildings | building							0
Square Footage (in sqft) | ft²									0
Gross Sales Price										200
Gain (Loss) on Sale										169
Disposed of by Sale | 4515 Seton Center Parkway
Real Estate Properties [Line Items]
Number of Properties | property															1
Number of Buildings | building															1
Square Footage (in sqft) | ft²															117,265
Disposed of by Sale | 4516 Seton Center Parkway
Real Estate Properties [Line Items]
Number of Properties | property															1
Number of Buildings | building															1
Square Footage (in sqft) | ft²															120,559
Disposed of by Sale | Seton Center
Real Estate Properties [Line Items]
Number of Properties | property															2
Number of Buildings | building															2
Square Footage (in sqft) | ft²															237,824
Gross Sales Price															$ 52,450
Gain (Loss) on Sale															$ 22,479
Disposed of by Sale | Cherrington Corporate Center
Real Estate Properties [Line Items]
Number of Properties | property								1
Number of Buildings | building							7
Square Footage (in sqft) | ft²									454,700
Disposed of by Sale | Foster Plaza
Real Estate Properties [Line Items]
Number of Properties | property								1
Number of Buildings | building							8
Square Footage (in sqft) | ft²									727,743
Disposed of by Sale | Pittsburgh Portfolio
Real Estate Properties [Line Items]
Number of Properties | property								2
Number of Buildings | building							15
Square Footage (in sqft) | ft²									1,182,443
Gross Sales Price										71,000
Gain (Loss) on Sale										$ (33,048)
Disposed of by Sale | 820 W. Diamond
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												1
Square Footage (in sqft) | ft²												134,933
Disposed of by Sale | Danac Stiles Business Park
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												3
Square Footage (in sqft) | ft²												276,637
Disposed of by Sale | 411 Farwell Avenue
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												1
Square Footage (in sqft) | ft²												422,727
Disposed of by Sale | 2250 Pilot Knob Road
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												1
Square Footage (in sqft) | ft²												87,183
Disposed of by Sale | 4700 Belleview Avenue
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												1
Square Footage (in sqft) | ft²												80,615
Disposed of by Sale | Five Property Portfolio
Real Estate Properties [Line Items]
Number of Properties | property												5
Number of Buildings | building												7
Square Footage (in sqft) | ft²												1,002,095
Gross Sales Price												$ 84,000
Gain (Loss) on Sale												$ 702