Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	6 Months Ended			0 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jul. 02, 2013 Series C	May 15, 2013 Series C	Feb. 15, 2013 Series C	Jul. 15, 2013 Series D	Apr. 15, 2013 Series D	Jan. 15, 2013 Series D	May 24, 2013 Common Shares	May 15, 2013 Common Shares item	Mar. 22, 2013 Common Shares	Feb. 22, 2013 Common Shares	Jul. 10, 2013 Distribution declared in July 2013
Distributions
Distribution to preferred shareholders (in dollars per share)					$ 0.4375	$ 0.4375	$ 0.4453125	$ 0.4453125	$ 0.4453125
Distribution declared per share (in dollars per share)														$ 0.47
Distribution to common shareholders (in dollars per share)										$ 0.47			$ 0.47
Common Share Issuances
Common stock, shares issued (in shares)												16,100,000
Common stock, issue price (in dollars per share)												$ 25.55
Net proceeds from sale of common shares	$ 393,474											$ 393,474
Common shares granted to trustees in period											2,000
Common stock granted per value per share	$ 0.01		$ 0.01
Common stock price per share which is the closing price at New York Stock Exchange (in dollars per share)											$ 30.69
Number of Trustees											5
Number of shares redeemed				6,700,000
Dividend rate (as a percent)				7.00%
Liquidation preference (in dollars per share)				$ 25.00
Recognized loss on the excess of liquidation preference of the redeemed shares over carrying amount		$ 2,944		$ 5,627