Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Earnings Per Share [Abstract]
Net increase (decrease) in shareholders' equity resulting from operations	$ 29,537		$ (1,160)		$ 72,360		$ 8,626
Net increase (decrease) in shareholders' equity resulting from operations - common shares	28,914		(1,134)		70,736		8,429
Net increase (decrease) in shareholders' equity resulting from operations - LTIPs	623		(26)		1,624		197
Dividends Paid, Common shares	(13,971)		(6,598)		(32,063)		(34,822)
Dividends Paid, LTIPs	(273)		(154)		(703)		(812)
Total dividends paid to common share and LTIP holders	(14,244)	[1]	(6,752)	[1]	(32,766)	[1]	(35,634)	[1]
Undistributed earnings, Common shares	14,943		(7,732)		38,673		(26,393)
Undistributed earnings, LTIPs	350		(180)		921		(615)
Total undistributed earnings attributable to common share and LTIP holders	$ 15,293		$ (7,912)		$ 39,594		$ (27,008)
Weighted average common shares outstanding	18,161,224		16,501,829		17,023,011		16,503,229
Weighted average participating LTIPs	391,604		385,652		390,871		385,220
Basic earnings per common share, Distributed	$ 0.77		$ 0.40		$ 1.88		$ 2.11
Basic earnings per common share, Undistributed	$ 0.82		$ (0.47)		$ 2.27		$ (1.60)
Basic earnings per common share	$ 1.59		$ (0.07)		$ 4.15		$ 0.51
Diluted earnings per common share, Distributed	$ 0.77		$ 0.40		$ 1.88		$ 2.11
Diluted earnings per common share, Undistributed	$ 0.82		$ (0.47)		$ 2.27		$ (1.60)
Diluted earnings per common share	$ 1.59		$ (0.07)		$ 4.15		$ 0.51

[1] For the three month periods ending September 30, 2012 and 2011, dividends totaling $0.70 and $0.40, respectively, per common share and LTIP unit outstanding were declared and paid. For the nine month periods ending September 30, 2012 and 2011, dividends totaling $1.80 and $2.11, respectively, per common share and LTIP unit outstanding were declared and paid.