Segment Information (Real Estate Assets by Segment) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Real estate assets	$ 5,260,295	$ 5,865,562
North America [Member] | Operating Segments [Member] | Carolinas Richmond [Member]
Segment Reporting Information [Line Items]
Real estate assets	482,736	446,348
North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Segment Reporting Information [Line Items]
Real estate assets	382,325	387,182
North America [Member] | Operating Segments [Member] | Florida [Member]
Segment Reporting Information [Line Items]
Real estate assets	476,046	815,712
North America [Member] | Operating Segments [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Real estate assets	495,817	483,432
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Real estate assets	1,234,090	1,096,195
North America [Member] | Operating Segments [Member] | Minnesota [Member]
Segment Reporting Information [Line Items]
Real estate assets	182,737	322,627
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Real estate assets	332,933	282,159
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Real estate assets	236,317	410,195
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Real estate assets	1,288,212	1,447,322
United Kingdom [Member] | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Real estate assets	$ 149,082	$ 174,390