Shareholders' Equity (Details) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Payments for Repurchase of Redeemable Preferred Stock			$ 168,272,000	$ 110,726,000
Adjustments for redemption/repurchase of preferred shares/units	0	(3,633,000)	(5,730,000)	(3,796,000)
Common shares, shares issued			16.9
Issuance of common shares			236,301,000	0
Payments for Commissions			4,800,000
6.95% Series M Cumulative Redeemable Preferred Shares [Member]
Preferred stock dividend rate, percentage			6.95%
Payments for Repurchase of Redeemable Preferred Stock			168,300,000
Adjustments for redemption/repurchase of preferred shares/units			(5,700,000)
Gross Proceeds [Member]
Issuance of common shares			$ 241,500,000