Stockholders' Equity (Details) (USD $)	3 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Sep. 30, 2011	Dec. 31, 2010
Class of Stock [Line Items]
Common stock, shares outstanding	147,627,247			147,627,247	140,199,105
At the market stock offering program, net proceeds				$ 439,037,000
Shares of Common Stock upon the exercise of options to purchase Common Stock by certain employees				312,711
Shares of its Common Stock in connection with the redemption of an equal number of OP Units				2,442,407
Dividend paid on common stock		$ 0.50	$ 0.50		$ 0.50
Dividends payable, amount per share	$ 0.50			$ 0.50
Dividend paid date		Jul. 29, 2011	Apr. 29, 2011		Jan. 28, 2011
Dividend shareholder of record date	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011		Dec. 31, 2010
Dividends payable, date declared	Sep. 16, 2011
Dividend payable date	Oct. 31, 2011
Initial ATM Program [Member]
Class of Stock [Line Items]
At the market stock offering program, shares issued				4,228,993
At the market stock offering program, gross proceeds				400,000,000
At the market stock offering program, net proceeds				394,700,000
At the market stock offering program, remained available for issuance				0
At the market stock offering program, aggregate value of common stock				400,000,000
At the market stock offering program maximum length of sale (years)				3
Second ATM Program [Member]
Class of Stock [Line Items]
At the market stock offering program, shares issued	431,223
At the market stock offering program, gross proceeds	44,900,000
At the market stock offering program, net proceeds	44,400,000
At the market stock offering program, remained available for issuance	555,100,000
At the market stock offering program, aggregate value of common stock				$ 600,000,000
At the market stock offering program maximum length of sale (years)				3