Segment Information - Depreciation and amortization of leasing transaction costs (Details) (Operating Segments [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	$ 54,903	$ 64,549	$ 61,618
UNITED KINGDOM | United Kingdom [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	177	172	269
North America [Member] | Carolinas [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	2,329	1,935	1,726
North America [Member] | Chicago Milwaukee [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	466	251	233
North America [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	3,988	3,200	2,608
North America [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	10,157	9,781	9,753
North America [Member] | Minnesota [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	4,478	5,797	5,132
North America [Member] | Orlando [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	3,116	3,037	2,804
North America [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	1,803	1,729	1,724
North America [Member] | Richmond Hampton Roads [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	4,278	4,156	4,157
North America [Member] | South Florida [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	2,824	2,979	3,083
North America [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	12,272	15,040	14,109
North America [Member] | Tampa [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	3,384	3,033	2,959
North America [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	$ 5,631	$ 13,439	$ 13,061