Real Estate (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Real Estate and Accumulated Depreciation [Line Items]
Real Estate Depreciation Expense	$ 140,600,000	$ 144,300,000	$ 147,300,000
Land and land improvements	900,501,000	852,785,000
Operating Buildings and Improvements, Gross	4,353,433,000	4,092,056,000
Gross Operating Real Estate	5,253,934,000	4,944,841,000
Real Estate Accumulated Depreciation	1,170,030,000	1,047,336,000	923,790,000	822,648,000
Industrial Properties [Member]
Real Estate and Accumulated Depreciation [Line Items]
Land and land improvements	445,721,000	401,849,000
Operating Buildings and Improvements, Gross	2,097,334,000	1,878,520,000
Gross Operating Real Estate	2,543,055,000	2,280,369,000
Real Estate Accumulated Depreciation	511,259,000	453,576,000
Office Buildings [Member]
Real Estate and Accumulated Depreciation [Line Items]
Land and land improvements	454,780,000	450,936,000
Operating Buildings and Improvements, Gross	2,256,099,000	2,213,536,000
Gross Operating Real Estate	2,710,879,000	2,664,472,000
Real Estate Accumulated Depreciation	$ 658,771,000	$ 593,760,000