Real Estate (Carrying Value of Properties) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate Investment Property, at Cost [Abstract]
Land and Land Improvements	$ 1,153,680	$ 1,060,065
Building and improvements	4,443,583	4,298,012
Accumulated Depreciation	940,613	847,106
Total Real Estate	5,597,263	5,358,077
Depreciation	148,900	166,800	$ 181,000
Industrial Property [Member]
Real Estate Investment Property, at Cost [Abstract]
Land and Land Improvements	1,001,454	914,428
Building and improvements	3,873,830	3,725,331
Accumulated Depreciation	814,301	719,658
Total Real Estate	4,875,284	4,639,759
Office Building [Member]
Real Estate Investment Property, at Cost [Abstract]
Land and Land Improvements	152,226	145,637
Building and improvements	569,753	572,681
Accumulated Depreciation	126,312	127,448
Total Real Estate	$ 721,979	$ 718,318