Shareholders' Equity (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended			12 Months Ended		1 Months Ended	12 Months Ended
	Jan. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2007 Series B Y	Dec. 31, 2011 Series B Y	Feb. 13, 2012 Series B	Dec. 31, 2010 Series B	Dec. 31, 2011 Series C Y	Dec. 31, 2010 Series C	Jan. 31, 2012 Series D	Dec. 31, 2011 Series D Y
Shareholders' Equity
Number of cumulative redeemable preferred shares						3,450,000		3,450,000	12,700,000	12,700,000
Distribution rate (in dollars per share)					25.00	2.21875			1.75			1.78125
Liquidation preference (in dollars per share)						$ 25.00			$ 25.00			$ 25.00
Aggregate liquidation preference (in dollars)						$ 86,250		$ 86,250	$ 317,500	$ 317,500		$ 290,000
Redemption price per share (in dollars per share)						$ 25.00			$ 25.00
Cash distributions paid or payable (in dollars per share)	$ 0.45	$ 1.80	$ 1.80	$ 0.00
Number of shares redeemed							3,450,000
Recognized loss on the excess of the liquidation preference of the redeemed shares over carrying amount						2,944
Shares sold						3,450,000		3,450,000	12,700,000	12,700,000	11,600,000
Selling price of shares (in dollars per share)											$ 25.00
Proceeds from sale of preferred shares											$ 280,365
Common Share Distributions
Cash distributions paid or payable (in dollars per share)	$ 0.45	$ 1.80	$ 1.80	$ 0.00
Distributions paid as percentage of ordinary income		100.00%	100.00%