Real Estate (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Real Estate and Accumulated Depreciation [Line Items]
Real Estate Depreciation Expense	$ 144,300,000	$ 147,300,000	$ 144,500,000
Land and land improvements	856,783,000	801,271,000
Operating Buildings and Improvements, Gross	4,117,179,000	3,902,891,000
Gross Operating Real Estate	4,973,962,000	4,704,162,000
Real Estate Accumulated Depreciation	1,062,230,000	938,044,000	836,146,000	733,688,000
Industrial Properties [Member]
Real Estate and Accumulated Depreciation [Line Items]
Land and land improvements	404,788,000	367,543,000
Operating Buildings and Improvements, Gross	1,897,253,000	1,731,557,000
Gross Operating Real Estate	2,302,041,000	2,099,100,000
Real Estate Accumulated Depreciation	465,695,000	409,863,000
Office Buildings [Member]
Real Estate and Accumulated Depreciation [Line Items]
Land and land improvements	451,995,000	433,728,000
Operating Buildings and Improvements, Gross	2,219,926,000	2,171,334,000
Gross Operating Real Estate	2,671,921,000	2,605,062,000
Real Estate Accumulated Depreciation	$ 596,535,000	$ 528,181,000