Real Estate (Tables)

12 Months Ended

Dec. 31, 2011

Real Estate [Abstract]

Real Estate and Accumulated Depreciation by Type [Table Text Block]

	Land	Building
	And Land	And		Accumulated
	Improvements	Improvements	Total	Depreciation
2011
Industrial properties	$404,788	$1,897,253	$2,302,041	$465,695
Office properties	451,995	2,219,926	2,671,921	596,535
2011 Total	$856,783	$4,117,179	$4,973,962	$1,062,230
2010
Industrial properties	$367,543	$1,731,557	$2,099,100	$409,863
Office properties	433,728	2,171,334	2,605,062	528,181
2010 Total	$801,271	$3,902,891	$4,704,162	$938,044

Real Estate Properties Sold [Table Text Block]

	Number of	Leaseable
Reportable Segment	Buildings	Square Feet	Gross Proceeds
			(in thousands)
Northeast
Southeastern PA	1	35,212	$	3,882
Lehigh/Central PA	32	1,422,501	124,000
Northeast - Other	2	91,698	11,351
Central	6	919,480	71,301
South	21	1,750,489	154,656
Total	62	4,219,380	$	365,190

2010 Sales

	Number of	Leaseable
Reportable Segment	Buildings	Square Feet	Gross Proceeds
			(in thousands)
Northeast
Southeastern PA	2	63,925	$	5,987
Lehigh/Central PA	2	146,800	7,216
Northeast - Other	1	39,151	5,100
Central	1	26,660	523
South	4	401,791	10,190
Total	10	678,327	$	29,016