Real Estate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Real Estate Investment Property, at Cost [Abstract]
Building and improvements	$ 5,131,648	$ 5,339,314
Land and Land Improvements	1,184,927	1,188,774
Accumulated Depreciation	1,148,928	1,182,129
Total Real Estate	6,316,575	6,528,088
Depreciation	181,000	179,100	$ 162,500
Industrial Property [Member]
Real Estate Investment Property, at Cost [Abstract]
Building and improvements	3,637,484	3,513,534
Land and Land Improvements	824,140	773,874
Accumulated Depreciation	698,456	620,967
Total Real Estate	4,461,624	4,287,408
Office Building [Member]
Real Estate Investment Property, at Cost [Abstract]
Building and improvements	1,494,164	1,825,780
Land and Land Improvements	360,787	414,900
Accumulated Depreciation	450,472	561,162
Total Real Estate	$ 1,854,951	$ 2,240,680