Segment Reporting Summary of Revenues (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of Reportable Segments			4
Rental and related revenue	$ 231,322	$ 221,655	$ 702,190	$ 646,842
General contractor and service fee revenue	59,739	62,807	185,072	161,004
Revenues	291,061	284,462	887,262	807,846
Disposal Group, Including Discontinued Operation, Revenue	454	11,140	2,415	39,807
Total Revenues Including Revenues Related To Discontinued Operations	291,515	295,602	889,677	847,653
Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	290,036	282,877	883,910	803,823
Operating Segments [Member] | Industrial [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Rental and related revenue	131,212	122,643	394,209	354,468
Operating Segments [Member] | Office [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Rental and related revenue	60,672	63,120	194,236	187,973
Operating Segments [Member] | Medical Office [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Rental and related revenue	36,715	32,542	104,979	94,571
Operating Segments [Member] | All Other Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Rental and related revenue	1,698	1,765	5,414	5,807
Operating Segments [Member] | Service Operations [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
General contractor and service fee revenue	59,739	62,807	185,072	161,004
Corporate, Non-Segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Rental and related revenue	1,025	1,585	3,352	4,023
Disposal Group, Including Discontinued Operation, Revenue	$ 454	$ 11,140	$ 2,415	$ 39,807