Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Net increase (decrease) in shareholders' equity resulting from operations
Net increase in shareholders’ equity resulting from operations	$ 24,786	$ 29,537	$ 10,768	$ 32,055	$ 1,701	$ (1,160)	$ (1,321)	$ 11,107	$ 97,146		$ 10,327		$ 40,571
Net increase in shareholders’ equity resulting from operations - common shares									95,064		10,090		39,411
Net increase in shareholders’ equity resulting from operations - LTIPs									2,082		237		1,160
Dividends Paid(1):
Common shares									(46,433)	[1]	(41,414)	[1]	(32,972)	[1]
LTIPs									(979)	[1]	(968)	[1]	(934)	[1]
Total dividends paid to common share and LTIP holders									(47,412)	[1],[2]	(42,382)	[1],[2]	(33,906)	[1],[2]
Undistributed earnings:
Common shares									48,631		(31,324)		6,439
LTIPs									1,103		(731)		226
Total undistributed earnings attributable to common share and LTIP holders									$ 49,734		$ (32,055)		$ 6,665
Weighted average shares outstanding (basic and diluted):
Weighted average common shares outstanding									17,897,996		16,497,983		12,958,969
Weighted average participating LTIPs									391,944		385,973		381,339
Basic earnings per common share:
Distributed (in usd per share)									$ 2.59		$ 2.51		$ 2.54
Undistributed (in usd per share)									$ 2.72		$ (1.90)		$ 0.50
Basic earnings per common share (in usd per share)									$ 5.31		$ 0.61		$ 3.04
Diluted earnings per common share:
Distributed (in usd per share)									$ 2.59		$ 2.51		$ 2.54
Undistributed (in usd per share)									$ 2.72		$ (1.90)		$ 0.50
Diluted earnings per common share (in usd per share)									$ 5.31		$ 0.61		$ 3.04

[1] The Company pays quarterly dividends in arrears, so a portion of the dividends paid in each calendar year relate to the prior year’s earnings.

[2] For the years ended December 31, 2012, 2011, and 2010, dividends totaling $ 2.50, $2.51, and $2.45, respectively, per common share and LTIP unit outstanding were declared and paid.