Segment Information (Depreciation and amortization of leasing transaction costs) (Details) - Operating Segments [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	$ 53,273	$ 60,075	$ 54,903
United Kingdom [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	259	292	177
North America [Member] | Carolinas Richmond [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	4,944	5,307	6,607
North America [Member] | Chicago Milwaukee [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	2,026	1,161	466
North America [Member] | Florida [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	6,605	9,407	9,324
North America [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	4,713	4,466	3,988
North America [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	12,262	11,641	10,157
North America [Member] | Minnesota [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	3,832	4,986	4,478
North America [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	2,706	2,220	1,803
North America [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	6,134	12,346	12,272
North America [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	$ 9,792	$ 8,249	$ 5,631