Equity, Capital and other Interests (Details Textuals) (USD $)	6 Months Ended
	Jun. 30, 2011	Jun. 16, 2011	Dec. 31, 2010
Equity, Capital and Other Interests (Textuals) [Abstract]
Acquired all of its partner's interest in 861 partially owned units	861
May sell up to 17.0 million Common Shares	17,000,000
Years Company can trade existing Common Shares in market at current prices as well as through negotiated transactions	3 years
Common Stock issued during period, shares	3,000,000
Common Shares average price for total consideration through the ATM program	$ 50.84
Total consideration through ATM program	$ 154,500,000
Reduction of paid in capital	5,600,000
Reduction in partially owned properties	3,000,000
Authorization to issue additional common shares	10,000,000
Common shares added to ATM program	5,700,000
Partially owned development projects acquired from outside partner	2
Authorized $464.6 million to be repurchased	464,600,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		12,980,741
Redemption value of approximately $357.7 million	438,141,000		383,540,000
100,000,000 authorized preferred shares	100,000,000		100,000,000
Preferred Shares of beneficial interest, par value	$ 0.01		$ 0.01
Annual dividend of $1.62 per share	$ 1.62
Depository shares in relation to Preferred shares	ten times
Value of depository shares in relation to liquidation value and dividend per share	50
Business Acquisition [Line Items]
Cost of acquiring properties	573,853,000
OPERATING PARTNERSHIP
Equity, Capital and Other Interests (Textuals) [Abstract]
Acquired all of its partner's interest in 861 partially owned units	861
Common Stock issued during period, shares	3,000,000
Common Shares average price for total consideration through the ATM program	$ 50.84
Total consideration through ATM program	154,500,000
OP units issued by operating partnership	3,000,000
Reduction of paid in capital	5,600,000
Reduction in partially owned properties	3,000,000
Authorization to issue additional common shares	10,000,000
Common shares added to ATM program	5,700,000
Partially owned development projects acquired from outside partner	2
Redemption value of approximately $357.7 million	416,334,000
Annual dividend of $1.62 per Unit	$ 1.62
Depository units in relation to Preference Units	ten times
Value of depository units in relation to liquidation value and dividend per unit	50
Business Acquisition [Line Items]
Cost of acquiring properties	8,600,000
Partially Owned Properties acquired [Member]
Business Acquisition [Line Items]
Cost of acquiring properties	$ 8,600,000