Shareholders' Equity (Details 3) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended
	Jan. 10, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jan. 31, 2012 Series D	Dec. 31, 2012 Series D	Dec. 31, 2011 Series D	Feb. 29, 2012 Series B	Dec. 31, 2012 Series B	Dec. 31, 2011 Series B	Sep. 30, 2012 Series C	Dec. 31, 2012 Series C	Dec. 31, 2011 Series C
Shareholders' Equity
Shares sold					11,600,000	11,600,000	0		0	3,450,000		6,700,000	12,700,000
Selling price of shares (in dollars per share)					$ 25.00
Proceeds from issuance of preferred shares, net		$ 280,107			$ 280,108
Distribution rate (in dollars per share)						$ 1.78125						$ 1.75
Liquidation preference (in dollars per share)						$ 25.00		$ 25.00			$ 25.00	$ 25.00
Aggregate liquidation preference (in dollars)						290,000	0		0	86,250		167,500	317,500
Redemption price per share (in dollars per share)						$ 25.00						$ 25.00
Number of shares redeemed								3,450,000			6,000,000
Shares outstanding						11,600,000	0		0	3,450,000		6,700,000	12,700,000
Dividend rate (as a percent)					7.125%			8.875%			7.00%
Recognized loss on the excess of the liquidation preference of the redeemed shares over carrying amount		$ 7,984							$ 2,944			$ 5,040
Common Share Distributions
Cash distributions paid or payable (in dollars per share)	$ 0.47	$ 1.82	$ 1.80	$ 1.80
Distributions paid as percentage of ordinary income		100.00%	100.00%	100.00%