Stockholders' Equity (Details) (USD $)	1 Months Ended													12 Months Ended
	Nov. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Jan. 31, 2011	Dec. 31, 2010		Oct. 31, 2010	May 31, 2010	Oct. 31, 2009	Aug. 31, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Issuance of common stock, value														$ 4,376,698,000	$ 1,054,755,000	$ 221,596,000
Public offering of common stock	40,530,000	[1]	49,680,000	[1]	32,200,000	[1]	26,910,000	8,269,000	[1]	13,225,000	6,900,000	5,000,000	4,313,000	149,320,000	28,394,000	9,313,000
At-the-Market Offering Program [Member]
Public offering, average price per share														$ 29.25	$ 29.13
Issuance of common stock, value														273,000,000	126,600,000
Public offering of common stock														9,400,000	4,400,000
Shares available for issuance														26,200,000
Dividend Reinvestment And Direct Stock Purchase Plan [Member]
Shares issued														500,000	7,700,000
Cash proceeds from shares issued														$ 14,900,000	$ 204,300,000
Shares available for issuance														4,700,000

[1] Shares of our common stock were sold to underwriters, who sold the shares in one or more transactions on the Nasdaq Global Select Market, in the over-the-counter market, through negotiated transactions or otherwise at market prices prevailing at the time of sale.