Preferred Shares of Beneficial Interest (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
	May. 08, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Class of Stock [Line Items]
Redemption of preferred shares				$ 150,000
Redemption costs of preferred shares		$ 9,687	$ 0	9,687	$ 0
Preferred stock value issued		$ 0		$ 0		$ 2
6.5% Series F Preferred Shares [Member]
Class of Stock [Line Items]
Redemption of preferred shares	$ 150,000
Redemption costs of preferred shares	$ 9,700
Preferred stock value issued						$ 150,000
Redemption price per depositary share (usd per share)						$ 25
Preferred shares dividend percentage				6.50%		6.50%
Liquidation value per share (usd per share)						$ 2,500
Liquidation preference value						$ 150,000
6.5% Series F Preferred Shares [Member] | Preferred Shares Issued At Discount [Member]
Class of Stock [Line Items]
Liquidation preference value						$ 64,300
Effective dividend yield on preferred shares						8.25%