Shareholders' Equity (Details) (USD $)	1 Months Ended	3 Months Ended			1 Months Ended	3 Months Ended			3 Months Ended	1 Months Ended
	Jan. 31, 2013	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 8.375% Series O Cumulative Redeemable Preferred Shares [Member]	Jan. 31, 2013 Secondary Offering [Member]	Mar. 31, 2013 At-the-market equity issuance [Member]	Mar. 31, 2013 Line of Credit [Member]	Dec. 31, 2012 Line of Credit [Member]	Mar. 31, 2013 At-the-market equity issuance [Member] Gross Proceeds [Member]	Jan. 31, 2013 Secondary Offering [Member] Gross Proceeds [Member]
Preferred stock dividend rate, percentage				8.375%
Payments for Repurchase of Redeemable Preferred Stock		$ 177,955,000	$ 168,272,000	$ 178,000,000
Adjustments for redemption of preferred shares/units		(5,932,000)	(5,730,000)	(5,900,000)
Common shares, shares issued					41,400,000	213,333
Stock Issued During Period, Value, Other	14.25
Issuance of common shares		574,685,000	146,969,000		571,900,000	2,800,000			3,000,000	590,000,000
Line of Credit Facility, Amount Outstanding							0	285,000,000
Payments for Commissions						$ 60,000