Description of Business and Significant Accounting Policies (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 acre sqft	Dec. 31, 2010	Dec. 31, 2009
Description of Business [Abstract]
In-service office, industrial and retail properties (in units)	303
Square footage of commercial property (in sq feet)	29,300,000
Rental residential units (in units)	96
Residential condominiums for sale (in units)	17
Undeveloped land suitable for future development (in acres)	586.0
Undeveloped land considered core holdings (in acres)	524
Office properties completed but not yet stabilized (in units)	1
Percentage of equity interest in joint ventures maximum	50.00%
Number of real estate properties not wholly owned (in units)	35
Number of rental residential properties under development not wholly owned (in units)	1
Area of undeveloped land not wholly owned (in acres)	11
Ownership interest in unconsolidated property directly owned by the Company	12.50%
Square footage of unconsolidated property directly owned by the Company (in sq feet)	261,000
Consolidated in-service office properties not wholly owned (in units)	5
Common Units of partnership owned (in shares)	72,200,000
Percentage of ownership of Common Units	95.10%	95.00%
Number of officers of the Company who are also limited partners	1
Number of directors of the Company who are also limited partners	2
Common Units of partnership not owned by the Company (in shares)	3,700,000
Number of common units required to convert to one share of common stock (in shares)	1
Common Stock par value	$ 0.01	$ 0.01
Number of trading days preceeding Redemption Notice Date (in days)	10
Common units redeemed for a like number of common shares of stock (in shares)	64,469
Real estate assets, depreciation expense	$ 120,812	$ 117,639	$ 115,603
Maturity of highly liquid investments (in months)	3
Percent of annual REIT taxable income required to pay to the Company's stockholders	90.00%
Customers with leases on wholly owned properties	1,722
Percent of annualized cash rental revenue derived from geographic locations	10.00%
Limit of consolidated revenue generated by any single customer	10.00%
Adjustment interval of variable debt interest rates, mimimum (in months)	1
Adjustment interval of variable debt interest rates, maximum (in months)	3
Building [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Average	40
Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Average	15
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Minimum	5
Property, Plant and Equipment, Useful Life, Maximum	7
Leaseholds and Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Minimum	3
Property, Plant and Equipment, Useful Life, Maximum	10