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Segment Information (Details) (USD $)
In Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2011
Jun. 30, 2010
Dec. 31, 2010
Real estate investments, at cost:          
Operating properties $ 4,885,041   $ 4,885,041   $ 4,834,111
Construction-in-progress 33,314   33,314   33,322
Land inventory 120,245   120,245   110,055
Total revenue 145,407 135,497 293,611 276,497  
Property operating expenses, real estate taxes and third party management expenses 56,575 52,894 118,501 111,362  
Net operating income 88,832 82,603 175,110 165,135  
Pennsylvania Suburbs [Member]
          
Real estate investments, at cost:          
Operating properties 1,210,274   1,210,274   1,199,957
Construction-in-progress 0   0   0
Land inventory 0   0   0
Total revenue 39,137 37,332 79,431 76,814  
Property operating expenses, real estate taxes and third party management expenses 13,572 12,866 30,366 29,622  
Net operating income 25,565 24,466 49,065 47,192  
Philadelphia CBD [Member]
          
Real estate investments, at cost:          
Operating properties 928,880   928,880   911,354
Construction-in-progress 0   0   0
Land inventory 0   0   0
Total revenue 30,573 18,412 62,423 36,962  
Property operating expenses, real estate taxes and third party management expenses 11,220 7,956 23,439 15,793  
Net operating income 19,353 10,456 38,984 21,169  
Metropolitan, D.C. [Member]
          
Real estate investments, at cost:          
Operating properties 1,366,282   1,366,282   1,359,776
Construction-in-progress 0   0   0
Land inventory 0   0   0
Total revenue 32,616 34,661 65,258 69,403  
Property operating expenses, real estate taxes and third party management expenses 12,190 11,996 24,514 24,370  
Net operating income 20,426 22,665 40,744 45,033  
New Jersey/ Delaware [Member]
          
Real estate investments, at cost:          
Operating properties 568,992   568,992   568,413
Construction-in-progress 0   0   0
Land inventory 0   0   0
Total revenue 21,098 22,647 42,429 47,940  
Property operating expenses, real estate taxes and third party management expenses 10,004 10,543 21,751 22,787  
Net operating income 11,094 12,104 20,678 25,153  
Richmond, Virginia [Member]
          
Real estate investments, at cost:          
Operating properties 306,243   306,243   294,406
Construction-in-progress 0   0   0
Land inventory 0   0   0
Total revenue 9,277 8,769 18,076 18,168  
Property operating expenses, real estate taxes and third party management expenses 3,623 3,165 7,011 6,824  
Net operating income 5,654 5,604 11,065 11,344  
Austin, Texas [Member]
          
Real estate investments, at cost:          
Operating properties 254,920   254,920   254,019
Construction-in-progress 0   0   0
Land inventory 0   0   0
Total revenue 7,637 7,966 15,750 15,965  
Property operating expenses, real estate taxes and third party management expenses 3,420 3,441 6,514 6,762  
Net operating income 4,217 4,525 9,236 9,203  
California [Member]
          
Real estate investments, at cost:          
Operating properties 249,450   249,450   246,186
Construction-in-progress 0   0   0
Land inventory 0   0   0
Total revenue 5,382 5,967 10,791 11,871  
Property operating expenses, real estate taxes and third party management expenses 2,821 2,956 5,515 5,684  
Net operating income 2,561 3,011 5,276 6,187  
Corporate [Member]
          
Real estate investments, at cost:          
Operating properties 0   0   0
Construction-in-progress 33,314   33,314   33,322
Land inventory 120,245   120,245   110,055
Total revenue (313) (257) (547) (626)  
Property operating expenses, real estate taxes and third party management expenses (275) (29) (609) (480)  
Net operating income $ (38) $ (228) $ 62 $ (146)