Shareholders' Equity (Details) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Payments for Commissions	$ 3,000,000
Other Preferred Stock Dividends and Adjustments	5,730,000	163,000
Payments for Repurchase of Redeemable Preferred Stock	168,272,000	2,096,000
Common shares, shares issued	10.8
Proceeds from Issuance of Common Stock	146,969,000	0
6.95% Series M Cumulative Redeemable Preferred Shares [Member]
Other Preferred Stock Dividends and Adjustments	5,700,000
Preferred stock dividend rate, percentage	6.95%
Payments for Repurchase of Redeemable Preferred Stock	$ 168,300,000