Share Transactions - Loss on Repurchase of Preferred Shares (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended			1 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	May 31, 2012	Apr. 30, 2012 Series C Preferred Shares	May 31, 2012 Series A Preferred Shares	May 31, 2012 Series B Preferred Shares	May 31, 2012 Series A and B Preferred Shares
Class of Stock [Line Items]
Shares issued - Series C			$ 0	$ 315,789		$ 325,000
Shares redeemed during period, value							200,000	125,000
Preferred shares, dividend rate (as a percent)						6.75%	8.00%	7.875%
Preferred shares, redemption price per share					$ 25.00
Loss on repurchase of preferred shares	$ 0	$ 10,612	$ 0	$ 10,612					$ 10,600