Real Estate Properties - Summary of Properties Sold (Details) $ in Thousands		1 Months Ended								3 Months Ended				9 Months Ended
	Jun. 27, 2014 property building	Sep. 30, 2015 USD ($) ft² property building	Aug. 31, 2015 USD ($) ft² property building	Jun. 30, 2015 USD ($) ft² property building	May. 31, 2015 USD ($) ft² property building	Apr. 30, 2015 USD ($) ft² property building	Mar. 31, 2015 USD ($) ft² property building	Jan. 31, 2015 USD ($) ft² property building	Jan. 31, 2013 property building	Sep. 30, 2015 USD ($) ft²	Jun. 30, 2015 USD ($) ft² property building	May. 31, 2015 ft² building	Sep. 30, 2014 USD ($) ft²	Sep. 30, 2015 USD ($) ft² property building	Sep. 30, 2014 USD ($) ft²
Real Estate Properties [Line Items]
Number of Properties | property									3
Number of Buildings									18
Square Footage (in sqft) | ft²		25,286,000								25,286,000			42,919,000	25,286,000	42,919,000
Gain (Loss) on Sale | $										$ 39,793			$ 0	$ 42,953	$ 0
333 Laurel Oak Drive
Real Estate Properties [Line Items]
Number of Buildings												3
1000 Vorhees Drive and 400 Laurel Oak Drive
Real Estate Properties [Line Items]
Number of Buildings												3
Disposed of by Sale
Real Estate Properties [Line Items]
Number of Properties | property														82
Number of Buildings														117
Square Footage (in sqft) | ft²		16,368,645								16,368,645				16,368,645
Gross Sales Price | $														$ 1,724,263
Gain (Loss) on Sale | $														$ 42,953
Disposed of by Sale | 333 Laurel Oak Drive
Real Estate Properties [Line Items]
Number of Buildings					2		1
Disposed of by Sale | 1000 Vorhees Drive and 400 Laurel Oak Drive
Real Estate Properties [Line Items]
Number of Buildings					2		1
Disposed of by Sale | Portfolio of Small Office and Industrial Assets
Real Estate Properties [Line Items]
Number of Properties | property					45
Number of Buildings					53
Square Footage (in sqft) | ft²					5,287,790							5,287,790
Gross Sales Price | $					$ 376,000
Gain (Loss) on Sale | $					$ (8,075)
Disposed of by Sale | AL, LA, NC, SC Office Portfolio
Real Estate Properties [Line Items]
Number of Properties | property				6
Number of Buildings				9
Square Footage (in sqft) | ft²				3,031,848							3,031,848
Gross Sales Price | $				$ 417,450
Gain (Loss) on Sale | $				$ 41,596
Disposed of by Sale | St. Louis Portfolio
Real Estate Properties [Line Items]
Number of Properties | property				2
Number of Buildings				2
Square Footage (in sqft) | ft²				165,098							165,098
Gross Sales Price | $				$ 14,300
Gain (Loss) on Sale | $				$ (2,349)
Disposed of by Sale | Australia Portfolio
Real Estate Properties [Line Items]
Number of Properties | property											11
Number of Buildings											11
Square Footage (in sqft) | ft²				1,758,525							1,758,525
Gross Sales Price | $											$ 232,955
Gain (Loss) on Sale | $											$ (47,851)
Disposed of by Sale | Upstate New York Portfolio
Real Estate Properties [Line Items]
Number of Properties | property			11
Number of Buildings			30
Square Footage (in sqft) | ft²			1,966,928
Gross Sales Price | $			$ 104,625
Gain (Loss) on Sale | $			$ (12,458)
Disposed of by Sale | Suburban property
Real Estate Properties [Line Items]
Number of Properties | property	13
Number of Buildings	41
Disposed of by Sale | Suburban property | 11350 North Meridian Street
Real Estate Properties [Line Items]
Number of Properties | property								1
Number of Buildings								1
Square Footage (in sqft) | ft²								72,264
Gross Sales Price | $								$ 4,200
Gain (Loss) on Sale | $								$ 766
Disposed of by Sale | Suburban property | 333 Laurel Oak Drive
Real Estate Properties [Line Items]
Number of Properties | property							0
Number of Buildings							1
Square Footage (in sqft) | ft²							27,164
Gross Sales Price | $							$ 2,450
Gain (Loss) on Sale | $							$ 251
Disposed of by Sale | Suburban property | 1921 E. Alton Avenue
Real Estate Properties [Line Items]
Number of Properties | property							1
Number of Buildings							1
Square Footage (in sqft) | ft²							67,846
Gross Sales Price | $							$ 14,533
Gain (Loss) on Sale | $							$ 4,850
Disposed of by Sale | Suburban property | 46 Inverness Center Parkway
Real Estate Properties [Line Items]
Number of Properties | property						0
Number of Buildings						0
Square Footage (in sqft) | ft²						0
Gross Sales Price | $						$ 2,000
Gain (Loss) on Sale | $						$ 1,857
Disposed of by Sale | Suburban property | Sorrento Valley Business Park
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				4
Square Footage (in sqft) | ft²				105,003							105,003
Gross Sales Price | $				$ 23,500
Gain (Loss) on Sale | $				$ 11,896
Disposed of by Sale | Suburban property | 7450 Campus Drive
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					77,411							77,411
Disposed of by Sale | Suburban property | 129 Worthington Ridge Road
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					227,500							227,500
Disposed of by Sale | Suburban property | 599 Research Parkway
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					48,249							48,249
Disposed of by Sale | Suburban property | 181 Marsh Hill Road
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					162,036							162,036
Disposed of by Sale | Suburban property | 101 Barnes Road
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					45,755							45,755
Disposed of by Sale | Suburban property | 15 Sterling Drive
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					173,015							173,015
Disposed of by Sale | Suburban property | 35 Thorpe Avenue
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					79,862							79,862
Disposed of by Sale | Suburban property | 50 Barnes Industrial Road North
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					154,255							154,255
Disposed of by Sale | Suburban property | 5-9 Barnes Industrial Road
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					38,006							38,006
Disposed of by Sale | Suburban property | 860 North Main Street
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					31,165							31,165
Disposed of by Sale | Suburban property | One Barnes Industrial Road South
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					30,170							30,170
Disposed of by Sale | Suburban property | Village Lane
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					2
Square Footage (in sqft) | ft²					58,185							58,185
Disposed of by Sale | Suburban property | 100 Northfield Drive
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					116,986							116,986
Disposed of by Sale | Suburban property | 905 Meridian Lake Drive
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					74,652							74,652
Disposed of by Sale | Suburban property | 1717 Deerfield Road
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					141,186							141,186
Disposed of by Sale | Suburban property | 1955 West Field Court
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					59,130							59,130
Disposed of by Sale | Suburban property | 5015 S. Water Circle
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					113,524							113,524
Disposed of by Sale | Suburban property | Adams Place
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					2
Square Footage (in sqft) | ft²					230,259							230,259
Disposed of by Sale | Suburban property | Cabot Business Park
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					2
Square Footage (in sqft) | ft²					252,755							252,755
Disposed of by Sale | Suburban property | 2300 Crown Colony Drive
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					45,974							45,974
Disposed of by Sale | Suburban property | Myles Standish Industrial Park
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					2
Square Footage (in sqft) | ft²					74,800							74,800
Disposed of by Sale | Suburban property | 340 Thompson Road
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					25,000							25,000
Disposed of by Sale | Suburban property | 6710 Oxon Hill
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					118,336							118,336
Disposed of by Sale | Suburban property | 8800 Queen Avenue South
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					280,822							280,822
Disposed of by Sale | Suburban property | 9800 Sherlard Parkway
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					46,765							46,765
Disposed of by Sale | Suburban property | Rosedale Corporate Plaza
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					3
Square Footage (in sqft) | ft²					149,116							149,116
Disposed of by Sale | Suburban property | 1000 Shelard Parkway
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					62,499							62,499
Disposed of by Sale | Suburban property | 1900 Meyer Drury Drive
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					65,225							65,225
Disposed of by Sale | Suburban property | 131-165 West Ninth Street
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					75,517							75,517
Disposed of by Sale | Suburban property | 7-9 Vreeland Road
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					155,891							155,891
Disposed of by Sale | Suburban property | 5 Paragon Drive
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					119,089							119,089
Disposed of by Sale | Suburban property | 1000 Vorhees Drive and 400 Laurel Oak Drive
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					2
Square Footage (in sqft) | ft²					125,415							125,415
Disposed of by Sale | Suburban property | 1601 Veterans Highway
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					63,608							63,608
Disposed of by Sale | Suburban property | Two Corporate Center Drive
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					291,230							291,230
Disposed of by Sale | Suburban property | 11311 Cornell Center Drive
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					93,413							93,413
Disposed of by Sale | Suburban property | 5300 Kings Island Drive
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					159,421							159,421
Disposed of by Sale | Suburban property | 3 Crown Point Court
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					73,987							73,987
Disposed of by Sale | Suburban property | 515 Pennsylvania Avenue
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					82,000							82,000
Disposed of by Sale | Suburban property | 443 Gulph Road
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					21,000							21,000
Disposed of by Sale | Suburban property | 4350 Northern Pike
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					503,885							503,885
Disposed of by Sale | Suburban property | Thunderbolt Place
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					2
Square Footage (in sqft) | ft²					100,505							100,505
Disposed of by Sale | Suburban property | 6160 Kempsville Circle
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					129,565							129,565
Disposed of by Sale | Suburban property | 448 Viking Drive
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					75,374							75,374
Disposed of by Sale | Suburban property | Inverness Center
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				4
Square Footage (in sqft) | ft²				475,882							475,882
Disposed of by Sale | Suburban property | 12655 Olive Boulevard
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				98,588							98,588
Disposed of by Sale | Suburban property | 1285 Fern Ridge Parkway
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				66,510							66,510
Disposed of by Sale | Suburban property | 310-314 Invermay Road
Real Estate Properties [Line Items]
Number of Properties | property						1
Number of Buildings						1
Square Footage (in sqft) | ft²						47,480
Disposed of by Sale | Suburban property | 253-293 George Town Road
Real Estate Properties [Line Items]
Number of Properties | property						1
Number of Buildings						1
Square Footage (in sqft) | ft²						143,914
Disposed of by Sale | Suburban property | 7 Modal Crescent
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				164,160							164,160
Disposed of by Sale | Suburban property | 71-93 Whiteside Road
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				303,488							303,488
Disposed of by Sale | Suburban property | 9-13 Titanium Court
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				69,664							69,664
Disposed of by Sale | Suburban property | 16 Rodborough Road
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				90,525							90,525
Disposed of by Sale | Suburban property | 22 Rodborough Road
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				43,427							43,427
Disposed of by Sale | Suburban property | 127-161 Cherry Lane
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				278,570							278,570
Disposed of by Sale | Suburban property | 44-46 Mandarin Street
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				226,718							226,718
Disposed of by Sale | Suburban property | 19 Leadership Way
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				76,714							76,714
Disposed of by Sale | Suburban property | Widewaters Parkway
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			8
Square Footage (in sqft) | ft²			514,241
Disposed of by Sale | Suburban property | 5062 Brittonfield Parkway
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			40,162
Disposed of by Sale | Suburban property | Woodcliff Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			6
Square Footage (in sqft) | ft²			516,760
Disposed of by Sale | Suburban property | Interstate Place
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			2
Square Footage (in sqft) | ft²			61,399
Disposed of by Sale | Suburban property | 1000 Pittsford - Victor Road
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			73,358
Disposed of by Sale | Suburban property | 1200 Pittsford - Victor Road
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			18,900
Disposed of by Sale | Suburban property | Corporate Crossing
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			5
Square Footage (in sqft) | ft²			216,126
Disposed of by Sale | Suburban property | Canal View Boulevard
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			3
Square Footage (in sqft) | ft²			118,375
Disposed of by Sale | Suburban property | 14 Classic Street
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			37,084
Disposed of by Sale | Suburban property | 251 Salina Meadows Parkway
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			65,617
Disposed of by Sale | CBD
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings	2
Disposed of by Sale | CBD | Illinois Center
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			2
Square Footage (in sqft) | ft²			2,090,162
Gross Sales Price | $			$ 376,000
Gain (Loss) on Sale | $			$ 26,929
Disposed of by Sale | CBD | 16th and Race Street
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			608,625
Gross Sales Price | $			$ 43,000
Gain (Loss) on Sale | $			$ 7,922
Disposed of by Sale | CBD | 185 Asylum Street
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings		1
Square Footage (in sqft) | ft²		868,395								868,395				868,395
Gross Sales Price | $		$ 113,250
Gain (Loss) on Sale | $		$ 17,619
Disposed of by Sale | CBD | 100 South Charles Street
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					159,616							159,616
Disposed of by Sale | CBD | 525 Park Street
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					75,636							75,636
Disposed of by Sale | CBD | 2501 20th Place South
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				125,722							125,722
Disposed of by Sale | CBD | 420 20th Street North
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				514,893							514,893
Disposed of by Sale | CBD | 701 Poydras Street
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				1,256,971							1,256,971
Disposed of by Sale | CBD | 300 North Greene Street
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				324,305							324,305
Disposed of by Sale | CBD | 1320 Main Street
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				334,075							334,075
Disposed of by Sale | CBD | 310-320 Pitt Street
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings				1
Square Footage (in sqft) | ft²				313,865							313,865
Disposed of by Sale | CBD | 110 W Fayette Street
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings			1
Square Footage (in sqft) | ft²			304,906
Foreclosure | CBD | 225 Water Street
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings					1
Square Footage (in sqft) | ft²					318,997							318,997
Gross Sales Price | $					$ 0
Gain (Loss) on Sale | $					$ 0