Segment Information Building Acquisitions and Dispositions (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2015 USD ($) ft² a bldg	Sep. 30, 2015 USD ($) ft² a bldg
Land, Buildings and Improvements [Member]
Dispositions [Abstract]
Number of Real Estate Properties Sold	6	36
Sold Real Estate Property, Square Feet | ft²	448,000	2,476,000
Proceeds from Sale of Property | $	$ 89,784	$ 248,116
Houston [Member] | Land and Land Improvements [Member]
Acquisitions [Abstract]
Land acquired, acres | a	139
Houston [Member] | Land, Buildings and Improvements [Member]
Acquisitions [Abstract]
Number of real estate properties acquired	3
Acquired Real Estate Property, Square Feet | ft²	921,000
Payments to Acquire Commercial Real Estate | $	$ 86,600
Minnesota [Member] | Land, Buildings and Improvements [Member]
Dispositions [Abstract]
Number of Real Estate Properties Sold	1	2
Sold Real Estate Property, Square Feet | ft²	135,000	223,000
Proceeds from Sale of Property | $	$ 19,900	$ 25,700
Richmond Hampton Roads [Member] | Land and Land Improvements [Member]
Dispositions [Abstract]
Sold Land, Acres | a		3.1
Richmond Hampton Roads [Member] | Land, Buildings and Improvements [Member]
Dispositions [Abstract]
Number of Real Estate Properties Sold	0	22
Sold Real Estate Property, Square Feet | ft²	0	1,319,000
Proceeds from Sale of Property | $	$ 0	$ 110,300	[1]
Southeastern PA [Member] | Land, Buildings and Improvements [Member]
Dispositions [Abstract]
Number of Real Estate Properties Sold	3	4
Sold Real Estate Property, Square Feet | ft²	250,000	333,000
Proceeds from Sale of Property | $	$ 63,500	$ 72,220
Tampa [Member] | Land, Buildings and Improvements [Member]
Dispositions [Abstract]
Number of Real Estate Properties Sold	1	1
Sold Real Estate Property, Square Feet | ft²	32,000	32,000
Proceeds from Sale of Property | $	$ 2,525	$ 2,525
Other Segments [Member] | Land, Buildings and Improvements [Member]
Dispositions [Abstract]
Number of Real Estate Properties Sold	1	7
Sold Real Estate Property, Square Feet | ft²	31,000	569,000
Proceeds from Sale of Property | $	$ 3,859	$ 37,371
Southern California and Houston [Member] | Land and Land Improvements [Member]
Acquisitions [Abstract]
Land acquired, acres | a		139
Southern California and Houston [Member] | Land, Buildings and Improvements [Member]
Acquisitions [Abstract]
Number of real estate properties acquired		4
Acquired Real Estate Property, Square Feet | ft²		1,300,000
Payments to Acquire Commercial Real Estate | $		$ 123,400

[1] Includes proceeds from the sale of 3.1 acres of land.