Segment Information (Depreciation and amortization of leasing transaction costs) (Details) - Operating Segments [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	$ 49,411	$ 53,273	$ 60,075
United Kingdom [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	339	259	292
North America [Member] | Carolinas Richmond [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	5,061	4,944	5,307
North America [Member] | Chicago Minneapolis [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	4,306	5,858	6,147
North America [Member] | Florida [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	4,084	6,605	9,407
North America [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	5,071	4,713	4,466
North America [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	14,371	12,262	11,641
North America [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	2,754	2,706	2,220
North America [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	3,582	6,134	12,346
North America [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	$ 9,843	$ 9,792	$ 8,249