Stockholders' Equity (Narrative) (Detail) (USD $)	1 Months Ended				9 Months Ended
	Sep. 17, 2012	Jul. 31, 2012	Apr. 30, 2012	Jan. 27, 2012	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Class of Stock [Line Items]
Common stock, shares outstanding					150,856,237		148,107,611
At the market stock offering program, shares issued					150,935,137		148,186,511
At the market stock offering program, net proceeds					$ 247,186,000	$ 439,037,000
Shares of Common Stock upon the exercise of options to purchase Common Stock by certain employees					22,823
Shares of its Common Stock in connection with the redemption of an equal number of OP Units					365,688
Dividend paid on common stock		$ 0.55	$ 0.55	$ 0.55
Dividend declared on common stock	$ 0.55
Quarter 4 [Member]
Class of Stock [Line Items]
Dividend record date					Dec. 31, 2011
Dividend payable date					Jan. 27, 2012
Quarter 1 [Member]
Class of Stock [Line Items]
Dividend record date					Mar. 30, 2012
Dividend payable date					Apr. 30, 2012
Quarter 2 [Member]
Class of Stock [Line Items]
Dividend record date					Jun. 29, 2012
Dividend payable date					Jul. 31, 2012
Quarter 3 [Member]
Class of Stock [Line Items]
Dividend record date					Sep. 28, 2012
Dividend payable date					Oct. 31, 2012
Dividend declared date					Sep. 17, 2012
ATM Program [Member]
Class of Stock [Line Items]
At the market stock offering program, aggregate value of common stock					600,000,000
At the market stock offering program, shares issued					2,347,500
At the market stock offering program, net proceeds					249,800,000
At the market stock offering program, net proceeds					247,200,000
At the market stock offering program remaining amount available for issuance					$ 305,300,000