Investment in Office Properties - Summary of 2013 Acquisitions (Details) (USD $)	12 Months Ended					0 Months Ended	12 Months Ended		0 Months Ended						1 Months Ended						0 Months Ended
	Dec. 31, 2012	Dec. 31, 2013 sqft Property	Mar. 09, 2012 sqft	Feb. 10, 2012	Dec. 31, 2011 Property	Dec. 19, 2013 Thomas Properties Group, Inc. [Member]	Dec. 31, 2013 Lincoln Place	Dec. 31, 2013 Office Building [Member] Property	Jan. 17, 2013 GEORGIA Office Building [Member] Tower Place 200 [Member] sqft	Mar. 07, 2013 FLORIDA Office Building [Member] Deerwood [Member] Property sqft	Dec. 06, 2013 FLORIDA Office Building [Member] Lincoln Place sqft	Aug. 28, 2013 ARIZONA Land Available for Development [Member] Land [Member] acre	Dec. 31, 2013 Mortgages [Member] FLORIDA Office Building [Member] Lincoln Place	Dec. 06, 2013 Mortgages [Member] FLORIDA Office Building [Member] Lincoln Place	Aug. 19, 2013 Management [Member] FLORIDA Land Available for Development [Member] Land [Member] acre	Aug. 19, 2013 Chief Executive Officer [Member] FLORIDA Land Available for Development [Member] Land [Member]	Aug. 19, 2013 ACP-Laurich Partnership Ltd [Member] FLORIDA Land Available for Development [Member] Land [Member]	Aug. 19, 2013 Executive Vice President [Member] FLORIDA Land Available for Development [Member] Land [Member]	Dec. 19, 2013 Wholly Owned Properties [Member] Thomas Properties Group, Inc. [Member] Property	Dec. 19, 2013 Partially Owned Properties [Member] Thomas Properties Group, Inc. [Member] Property	Dec. 19, 2013 Commerce Square [Member] PENNSYLVANIA Office Building [Member] Property sqft	Dec. 19, 2013 Four Points Centre [Member] TEXAS Office Building [Member] sqft Property
Business Acquisition [Line Items]
Area of real estate property (in square feet)		17,600,000	337,000						258,000	1,000,000	140,000
Total Purchase Price	$ 898,400,000					$ 1,238,479,000			$ 56,300,000	$ 130,000,000		$ 1,200,000			$ 2,900,000
Number of real estate properties		43			2			50		2									2	5	2	2
Net rentable area (in square feet)																					1,900,000	194,000
Asset disposal gross sale price																						47,300,000
Proceeds from sale of real estate																					71,800,000	21,700,000
Mortgage assumed		58,694,000				335,604,000
Mortgage notes payable	$ 605,889,000	$ 1,097,493,000								$ 84,500,000			$ 49,300,000	$ 49,300,000
Interest rate on mortgage (in percent)				1.50%						3.90%				5.90%
Area of land												1			6
Number of common units issued in a business acquisition							4,400,000				900,000					11,966	29,916	5,983