Segment Information - Depreciation and amortization of leasing transaction costs (Details) (Operating Segments [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	$ 65,222	$ 61,618	$ 64,028
European Union [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	845	269	210
North America [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	9,781	9,753	9,821
North America [Member] | Chicago Milwaukee [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	251	233	1,695
North America [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	3,200	2,608	2,213
North America [Member] | Carolinas [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	1,935	1,726	2,194
North America [Member] | Industrial Other [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	10,061	10,461	11,254
North America [Member] | Minnesota [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	5,797	5,132	5,014
North America [Member] | South Florida [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	2,979	3,083	3,623
North America [Member] | Richmond Hampton Roads [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	4,156	4,157	4,820
North America [Member] | Arizona [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	3,546	3,014	2,694
North America [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	5,654	5,193	5,217
North America [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	1,729	1,724	1,774
North America [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	15,040	14,109	13,436
North America [Member] | Washington DC [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization of tenant improvement and lease transaction costs	$ 248	$ 156	$ 63