Real Estate Properties Sold (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disposition Disclosures [Line Items]
Proceeds from Sale of Real Estate Held-for-investment	$ 559,322	$ 526,949	$ 234,686
Building and Building Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of Real Estate Properties Sold	65	58
Sold Real Estate Property, Square Feet	4,740,329	5,123,172
Proceeds from Sale of Real Estate Held-for-investment	558,578	491,780
Building and Building Improvements [Member] | Houston [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of Real Estate Properties Sold	5
Sold Real Estate Property, Square Feet	553,510
Proceeds from Sale of Real Estate Held-for-investment	29,721
Building and Building Improvements [Member] | Lehigh Central PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of Real Estate Properties Sold		1
Sold Real Estate Property, Square Feet		101,750
Proceeds from Sale of Real Estate Held-for-investment		8,650
Building and Building Improvements [Member] | Carolinas [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of Real Estate Properties Sold		1
Sold Real Estate Property, Square Feet		59,200
Proceeds from Sale of Real Estate Held-for-investment		2,000
Building and Building Improvements [Member] | Minnesota [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of Real Estate Properties Sold	1	12
Sold Real Estate Property, Square Feet	191,336	915,693
Proceeds from Sale of Real Estate Held-for-investment	41,000	71,493
Building and Building Improvements [Member] | Orlando [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of Real Estate Properties Sold	4
Sold Real Estate Property, Square Feet	588,678
Proceeds from Sale of Real Estate Held-for-investment	116,750
Building and Building Improvements [Member] | Richmond Hampton Roads [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of Real Estate Properties Sold		1
Sold Real Estate Property, Square Feet		86,170
Proceeds from Sale of Real Estate Held-for-investment		10,978
Building and Building Improvements [Member] | Philadelphia [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of Real Estate Properties Sold		2
Sold Real Estate Property, Square Feet		656,350
Proceeds from Sale of Real Estate Held-for-investment		103,650
Building and Building Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of Real Estate Properties Sold	3	5
Sold Real Estate Property, Square Feet	113,833	996,874
Proceeds from Sale of Real Estate Held-for-investment	12,225	102,734
Building and Building Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of Real Estate Properties Sold	2	36
Sold Real Estate Property, Square Feet	134,510	2,307,135
Proceeds from Sale of Real Estate Held-for-investment	8,655	192,275
Maryland [Member] | Building and Building Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of Real Estate Properties Sold	23
Sold Real Estate Property, Square Feet	1,367,569
Proceeds from Sale of Real Estate Held-for-investment	185,460
New Jersey [Member] | Building and Building Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of Real Estate Properties Sold	27
Sold Real Estate Property, Square Feet	1,790,893
Proceeds from Sale of Real Estate Held-for-investment	$ 164,767