Stockholders' Equity (Details) (USD $)	1 Months Ended		3 Months Ended						1 Months Ended			1 Months Ended
	Apr. 30, 2012	Feb. 29, 2012	Mar. 31, 2012		Mar. 31, 2011		Mar. 31, 2012 DRIP [Member]	Mar. 31, 2011 DRIP [Member]	Feb. 29, 2012 Series D Depositary Share [Member]	Mar. 31, 2012 Series D Depositary Share [Member]	Dec. 31, 2011 Series D Depositary Share [Member]	Mar. 31, 2012 Series C Depositary Share [Member]	Dec. 31, 2011 Series C Depositary Share [Member]
Class of Stock [Line Items]
Dividends, Preferred Stock			$ 1,979,000	[1]	$ 1,696,000	[1]
Preferred Stock, Dividends, Per Share, Cash Paid			$ 0.5378	[1]	$ 0.4609	[1]
Dividends, Common Stock			40,432,000		31,678,000
Common Stock, Dividends, Per Share, Cash Paid			$ 0.385		$ 0.380
Preferred Stock, Shares Issued									11,500,000	11,500,000	0	0	3,680,000
Preferred Stock, Dividend Rate, Percentage									6.625%			7.375%
Preferred Stock, Redemption Price Per Share									$ 25.00			$ 25.00
Proceeds from issuance of preferred stock			287,500,000		0				277,645,000
Stock Issuance Costs									9,855,000
Stock Redeemed or Called During Period, Shares												(3,680,000)
Preferred Stock, Fixed Annual Dividend Per Share, Depository Shares												$ 25.0768229
Preferred Stock Redemption Premium												3,098,000
Common Stock, Dividends, Per Share, Declared	$ 0.385
Common Stock, Shares Reserved for Future Issuance, Dividend Reinvestment Plan		16,000,000
Stock Issued During Period, Shares, Dividend Reinvestment Plan							1,428,069	818,156
Proceeds from issuance of common stock			$ 37,550,000		$ 20,025,000		$ 37,666,000	$ 20,081,000

[1] The Series C preferred stock was redeemed in March 2012. The dividends paid during the quarter ended March 31, 2012 include accumulated and unpaid dividends through the redemption date.