Real Estate Properties - Summary of Properties Sold (Details) $ in Thousands	1 Months Ended								3 Months Ended		9 Months Ended		12 Months Ended
	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 ($) ft² building property	Jan. 31, 2013 building property	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) ft² building property	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) ft² building
Real Estate Properties [Line Items]
Number of Properties | property								3
Number of Buildings | building								18
Gain (Loss) on Sale									$ 82,169	$ 39,793	$ 225,210	$ 42,953
Disposed of by Sale
Real Estate Properties [Line Items]
Number of Properties | property											27
Number of Buildings | building											56		135
Square Footage (in sqft) | ft²											7,222,691		18,900,000
Gross Sales Price											$ 1,142,365		$ 2,000,000
Gain (Loss) on Sale											$ 225,095
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						$ 14,677
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(1)
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		$ 78,240
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		$ 4,745
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 | 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		$ 7,244
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	$ (15,808)