Shareholders' Equity (Details) (USD $) Share data in Millions, unless otherwise specified	1 Months Ended	3 Months Ended		6 Months Ended			1 Months Ended	6 Months Ended		6 Months Ended	1 Months Ended
	Jan. 31, 2013	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 8.375% Series O Cumulative Redeemable Preferred Shares [Member]	Jan. 31, 2013 Secondary Offering [Member]	Jun. 30, 2013 At-the-market equity issuance [Member]	Dec. 31, 2012 Line of Credit [Member]	Jun. 30, 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		0	0	(5,932,000)	(5,730,000)	(5,900,000)
Common shares, shares issued							41.4	1.7
Stock Issued During Period, Value, Other	14.25
Issuance of common shares				601,927,000	151,258,000		571,900,000	30,100,000		30,900,000	590,000,000
Line of Credit Facility, Amount Outstanding									285,000,000
Payments for Commissions								$ 587,000