Description of Business and Significant Accounting Policies (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 executives D property sqft acre properties	Dec. 31, 2011
Organization, Consolidation and Presentation of Financial Statements [Abstract]
In-service office, industrial and retail properties (in units)	303
Square footage of commercial property (in sq feet)	29,296,000
Residential condominiums for sale (in units)	14
Undeveloped land suitable for future development (in acres)	581
Undeveloped land considered core holdings (in acres)	518
Office properties planned for development (in units)	1
Office properties completed but not yet stabilized (in units)	1
Common Units of partnership owned (in shares)	73,200,000
Percentage of ownership of Common Units	95.20%	95.10%
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)	2,000
Number of Common Stock sold at the market during period (in shares)	785,500
Average price of Common Stock sold at the market during period (in dollars per share)	$ 32.82
Net proceeds of Common Stock sold at the market during period	$ 25.4