Subsequent Events (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended	3 Months Ended	12 Months Ended	0 Months Ended
	Apr. 11, 2012 6.90% Series E Cumulative Redeemable Preferred Shares Issuance of Equity	Mar. 31, 2012 7.50% Series C Preferred Shares	Dec. 31, 2011 7.50% Series C Preferred Shares	May 03, 2012 7.50% Series C Preferred Shares Repurchase of Equity
Subsequent Event [Line Items]
Issuance of Preferred shares (in shares)	4,000,000
Preferred shares, dividend rate (as a percent)	6.90%	7.50%	7.50%	7.50%
Preferred shares, par value (in dollars per share)	$ 25	$ 0.01	$ 0.01
Preferred shares, liquidation preference (in dollars per share)	$ 25
Preferred Stock, per share per annum (in dollars per share per annum)	$ 1.725
Proceeds from issuance of redeemable preferred shares	$ 96.3
Shares redeemed (in shares)				2,000,000
Shares redeemed				50.0
Dividends Payable, Amount				$ 0.2