Real Estate Properties - Summary of Properties Disposed (Details) $ in Thousands	1 Months Ended																					12 Months Ended
	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 ($) ft² building property	Dec. 31, 2016 USD ($) ft² building property	Nov. 30, 2016 USD ($) ft² building property	Oct. 31, 2016 USD ($) ft² building property	Aug. 31, 2016 USD ($) ft² building property	Jul. 31, 2016 USD ($) ft² building property	Jun. 30, 2016 USD ($) ft² building property	May 31, 2016 USD ($) ft² building property	Apr. 30, 2016 USD ($) ft² building property	Mar. 31, 2016 USD ($) ft² building property	Feb. 29, 2016 USD ($) ft² building property	Jan. 31, 2013 building	Dec. 31, 2017 USD ($) ft² building property	Dec. 31, 2016 USD ($) ft² building property	Dec. 31, 2015 USD ($) building
Real Estate Properties [Line Items]
Number of Buildings | building																					18
Gain (Loss) on Sale																						$ 15,498	$ 250,886	$ 84,421
Disposed of by Sale
Real Estate Properties [Line Items]
Number of Properties | property																						16	30
Number of Buildings | building																						37	62	135
Square Footage (in sqft) | ft²																						6,588,128	7,972,692
Gross Sales Price																						$ 862,624	$ 1,259,895	$ 2,000,000
Gain (Loss) on Sale																						$ 15,493	$ 250,761
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
Disposed of by Sale | Executive Park
Real Estate Properties [Line Items]
Number of Properties | property																				1
Number of Buildings | building																				9
Square Footage (in sqft) | ft²																				427,443
Gross Sales Price																				$ 50,865
Gain (Loss) on Sale																				$ 16,531
Disposed of by Sale | 3330 N Washington Boulevard
Real Estate Properties [Line Items]
Number of Properties | property																			1
Number of Buildings | building																			1
Square Footage (in sqft) | ft²																			55,719
Gross Sales Price																			$ 11,250
Gain (Loss) on Sale																			$ 5,455
Disposed of by Sale | 111 East Kilbourn Avenue
Real Estate Properties [Line Items]
Number of Properties | property																			1
Number of Buildings | building																			1
Square Footage (in sqft) | ft²																			373,669
Gross Sales Price																			$ 60,500
Gain (Loss) on Sale																			$ 14,687
Disposed of by Sale | 1525 Locust Street
Real Estate Properties [Line Items]
Number of Properties | property																		1
Number of Buildings | building																		1
Square Footage (in sqft) | ft²																		98,009
Gross Sales Price																		$ 17,700
Gain (Loss) on Sale																		$ 8,956
Disposed of by Sale | 633 Ahua Street
Real Estate Properties [Line Items]
Number of Properties | property																		1
Number of Buildings | building																		1
Square Footage (in sqft) | ft²																		93,141
Gross Sales Price																		$ 29,000
Gain (Loss) on Sale																		$ 15,963
Disposed of by Sale | Lakewood on the Park
Real Estate Properties [Line Items]
Number of Properties | property																	1
Number of Buildings | building																	2
Square Footage (in sqft) | ft²																	180,558
Gross Sales Price																	$ 37,100
Gain (Loss) on Sale																	$ 13,616
Disposed of by Sale | Leased Land
Real Estate Properties [Line Items]
Number of Properties | property																1
Number of Buildings | building																7
Square Footage (in sqft) | ft²																0
Gross Sales Price																$ 48,450
Gain (Loss) on Sale																$ 15,914
Disposed of by Sale | 9110 East Nichols Avenue
Real Estate Properties [Line Items]
Number of Properties | property																1
Number of Buildings | building																1
Square Footage (in sqft) | ft²																143,958
Gross Sales Price																$ 17,200
Gain (Loss) on Sale																$ 642
Disposed of by Sale | 111 River Street
Real Estate Properties [Line Items]
Number of Properties | property															1
Number of Buildings | building															1
Square Footage (in sqft) | ft²															566,215
Gross Sales Price															$ 235,000
Gain (Loss) on Sale															$ 78,207
Disposed of by Sale | Sky Park Centre
Real Estate Properties [Line Items]
Number of Properties | property															1
Number of Buildings | building															2
Square Footage (in sqft) | ft²															63,485
Gross Sales Price															$ 13,700
Gain (Loss) on Sale															$ 4,746
Disposed of by Sale | Raintree Industrial Park
Real Estate Properties [Line Items]
Number of Properties | property															1
Number of Buildings | building															12
Square Footage (in sqft) | ft²															563,182
Gross Sales Price															$ 11,500
Gain (Loss) on Sale															$ (653)
Disposed of by Sale | 8701 N Mopac
Real Estate Properties [Line Items]
Number of Properties | property														1
Number of Buildings | building														1
Square Footage (in sqft) | ft²														121,901
Gross Sales Price														$ 21,500
Gain (Loss) on Sale														$ 8,394
Disposed of by Sale | 7800 Shoal Creek Boulevard
Real Estate Properties [Line Items]
Number of Properties | property													1
Number of Buildings | building													4
Square Footage (in sqft) | ft²													151,917
Gross Sales Price													$ 29,210
Gain (Loss) on Sale													$ 14,908
Disposed of by Sale | 1200 Lakeside Drive
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												1
Square Footage (in sqft) | ft²												260,084
Gross Sales Price												$ 65,270
Gain (Loss) on Sale												$ 3,062
Disposed of by Sale | 6200 Glenn Carlson Drive
Real Estate Properties [Line Items]
Number of Properties | property											1
Number of Buildings | building											1
Square Footage (in sqft) | ft²											338,000
Gross Sales Price											$ 23,050
Gain (Loss) on Sale											$ 7,706
Disposed of by Sale | 812 San Antonio Street
Real Estate Properties [Line Items]
Number of Properties | property																	1
Number of Buildings | building																	1
Square Footage (in sqft) | ft²																	59,321
Disposed of by Sale | 1601 Rio Grande Street
Real Estate Properties [Line Items]
Number of Properties | property																	1
Number of Buildings | building																	1
Square Footage (in sqft) | ft²																	56,219
Disposed of by Sale | Downtown Austin portfolio
Real Estate Properties [Line Items]
Number of Properties | property																	2
Number of Buildings | building																	2
Square Footage (in sqft) | ft²																	115,540
Gross Sales Price																	$ 32,600
Gain (Loss) on Sale																	$ 20,584
Disposed of by Sale | 785 Schilinger Road South
Real Estate Properties [Line Items]
Number of Properties | property																1
Number of Buildings | building																1
Square Footage (in sqft) | ft²																72,000
Disposed of by Sale | 401 Vine Street
Real Estate Properties [Line Items]
Number of Properties | property																1
Number of Buildings | building																1
Square Footage (in sqft) | ft²																53,980
Disposed of by Sale | 633 Frazier Drive
Real Estate Properties [Line Items]
Number of Properties | property																1
Number of Buildings | building																1
Square Footage (in sqft) | ft²																150,000
Disposed of by Sale | 9840 Gateway Boulevard North
Real Estate Properties [Line Items]
Number of Properties | property																1
Number of Buildings | building																1
Square Footage (in sqft) | ft²																72,000
Disposed of by Sale | 3003 South Expressway 281
Real Estate Properties [Line Items]
Number of Properties | property																1
Number of Buildings | building																1
Square Footage (in sqft) | ft²																150,000
Disposed of by Sale | 1331 North Center Parkway
Real Estate Properties [Line Items]
Number of Properties | property																1
Number of Buildings | building																1
Square Footage (in sqft) | ft²																53,980
Disposed of by Sale | Movie theater portfolio
Real Estate Properties [Line Items]
Number of Properties | property																6
Number of Buildings | building																6
Square Footage (in sqft) | ft²																551,960
Gross Sales Price																$ 109,100
Gain (Loss) on Sale																$ 30,595
Disposed of by Sale | 128 Crews Drive
Real Estate Properties [Line Items]
Number of Properties | property															1
Number of Buildings | building															1
Square Footage (in sqft) | ft²															185,600
Disposed of by Sale | 111 Southchase Boulevard
Real Estate Properties [Line Items]
Number of Properties | property															1
Number of Buildings | building															1
Square Footage (in sqft) | ft²															168,087
Disposed of by Sale | 1043 Global Avenue
Real Estate Properties [Line Items]
Number of Properties | property															1
Number of Buildings | building															1
Square Footage (in sqft) | ft²															450,000
Disposed of by Sale | South Carolina industrial portfolio
Real Estate Properties [Line Items]
Number of Properties | property															3
Number of Buildings | building															3
Square Footage (in sqft) | ft²															803,687
Gross Sales Price															$ 30,000
Gain (Loss) on Sale															$ 7,248
Disposed of by Sale | 100 East Wisconsin Avenue
Real Estate Properties [Line Items]
Number of Properties | property														1
Number of Buildings | building														1
Square Footage (in sqft) | ft²														435,067
Disposed of by Sale | 111 Monument Circle
Real Estate Properties [Line Items]
Number of Properties | property														1
Number of Buildings | building														2
Square Footage (in sqft) | ft²														1,121,764
Disposed of by Sale | 101-115 W. Washington Street
Real Estate Properties [Line Items]
Number of Properties | property														1
Number of Buildings | building														1
Square Footage (in sqft) | ft²														634,058
Disposed of by Sale | North Point Office Complex
Real Estate Properties [Line Items]
Number of Properties | property														1
Number of Buildings | building														2
Square Footage (in sqft) | ft²														873,335
Disposed of by Sale | Midwest portfolio
Real Estate Properties [Line Items]
Number of Properties | property														4
Number of Buildings | building														6
Square Footage (in sqft) | ft²														3,064,224
Gross Sales Price														$ 416,900
Gain (Loss) on Sale														$ (15,800)