Segment Information - Depreciation and amortization of leasing transaction costs (Details) - Operating Segments [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	$ 60,075	$ 54,903	$ 64,549
UNITED KINGDOM | United Kingdom [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	292	177	172
North America [Member] | Carolinas [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	2,413	2,329	1,935
North America [Member] | Chicago Milwaukee [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	1,161	466	251
North America [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	4,466	3,988	3,200
North America [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	11,641	10,157	9,781
North America [Member] | Minnesota [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	4,986	4,478	5,797
North America [Member] | Orlando [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	1,963	3,116	3,037
North America [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	2,220	1,803	1,729
North America [Member] | Richmond Hampton Roads [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	2,894	4,278	4,156
North America [Member] | South Florida [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	3,572	2,824	2,979
North America [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	12,346	12,272	15,040
North America [Member] | Tampa [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	3,872	3,384	3,033
North America [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	$ 8,249	$ 5,631	$ 13,439