Real Estate (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Real Estate Investment Property, at Cost [Abstract]
Land and Land Improvements	$ 1,139,455,000	$ 845,958,000
Building and improvements	5,144,758,000	4,045,385,000
Total Real Estate Investment Property, at Cost	6,284,213,000	4,891,343,000
Accumulated Depreciation	1,057,680,000	1,072,859,000
Depreciation	162,500,000	140,600,000	144,300,000
Industrial Property [Member]
Real Estate Investment Property, at Cost [Abstract]
Land and Land Improvements	708,484,000	442,814,000
Building and improvements	3,238,875,000	2,064,928,000
Total Real Estate Investment Property, at Cost	3,947,359,000	2,507,742,000
Accumulated Depreciation	514,109,000	496,208,000
Office Building [Member]
Real Estate Investment Property, at Cost [Abstract]
Land and Land Improvements	430,971,000	403,144,000
Building and improvements	1,905,883,000	1,980,457,000
Total Real Estate Investment Property, at Cost	2,336,854,000	2,383,601,000
Accumulated Depreciation	$ 543,571,000	$ 576,651,000