Shareholders' Equity (Details) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Payments for Repurchase of Redeemable Preferred Stock			$ 168,272,000	$ 2,096,000
Adjustments for redemption/repurchase of preferred shares	0	0	(5,730,000)	(163,000)
Common shares, shares issued			11.1
Issuance of common shares			151,258,000	0
Payments for Commissions			3,100,000
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			(5,700,000)
Gross Proceeds [Member]
Issuance of common shares			$ 154,500,000