Stockholders' Equity (Details) (USD $)	0 Months Ended	3 Months Ended				0 Months Ended	1 Months Ended	3 Months Ended
	Jan. 27, 2012	Mar. 31, 2012	Mar. 31, 2011	Mar. 16, 2012	Dec. 31, 2011	May 08, 2012 Atm Program [Member]	May 08, 2012 Atm Program [Member]	Mar. 31, 2012 Atm Program [Member]	Mar. 31, 2012 Quarter 4 [Member]	Mar. 31, 2012 Quarter 4 [Member] OP Units [Member]	Mar. 31, 2012 Quarter 1 [Member]	Mar. 31, 2012 Quarter 1 [Member] OP Units [Member]
Class of Stock [Line Items]
Common stock, shares outstanding		149,384,341			148,107,611
Dividend paid on common stock	$ 0.55
Dividend shareholder of record date									Dec. 31, 2011		Mar. 30, 2012
Dividends payable, date declared											Mar. 16, 2012	Mar. 16, 2012
Dividends payable, amount per share				$ 0.55
Dividend payable date										Jan. 27, 2012		Apr. 30, 2012
At the market stock offering program, shares issued		149,463,241			148,186,511	643,600	643,600	1,048,800
At the market stock offering program remaining amount available for issuance						$ 376,200,000		$ 444,600,000
At the market stock offering program, aggregate value of common stock								600,000,000
At the market stock offering program, gross proceeds							68,400,000	110,500,000
At the market stock offering program, net proceeds		$ 109,330,000	$ 345,369,000				$ 67,700,000	$ 109,300,000
Shares of its Common Stock in connection with the redemption of an equal number of OP Units		199,526
Shares of Common Stock upon the exercise of options to purchase Common Stock by certain employees		17,706