Stockholders' Equity (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011		Sep. 30, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Series C preferred stock: Dividends	$ 1,696,000	$ 1,696,000	$ 5,089,000	[1]	$ 5,089,000	[1]
Series C preferred stock: Per share			$ 1.3828	[1]	$ 1.3828	[1]
Common stock: Dividends			96,915,000		93,762,000
Common stock: Per share	$ 26.07		$ 1.145		$ 1.130
Dividends declared			$ 0.385
Dividends payable date	2011-11		2011-11
Dividends recorded date			Oct. 31, 2011
Underwriters' over allotment of shares	1,200,000
Net proceeds	229,451,000		299,528,000		14,220,000
Issuance of common stock, shares	9,200,000
Stock issuance costs	10,393,000		10,224,000
DRIP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock issued			2,423,611		663,741
Net proceeds			$ 59,707,000		$ 14,159,000

[1] The Series C preferred stock has no maturity date and will remain outstanding unless redeemed.