Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014		Dec. 31, 2013
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 1,780	$ 3,896	$ 13,152	$ 19,261	$ 2,637	$ 12,948	$ 20,947	$ 22,635	$ 38,089	$ 59,167	[1]	$ 78,524	[1]
Net Income (Loss) Attributable to Participating Non-controlling Interest	[2]									239	472		735
Net income (loss) attributable to parent and participating non-controlling interest										38,328	59,639		79,259
Net increase in shareholders' equity resulting from operations– common shares										37,604	58,306		77,219
Net increase in shareholders' equity resulting from operations– LTIP units										485	861		1,305
Dividends Paid:
Common shareholders	[3]									(81,886)	(84,527)		(89,732)
LTIP unit holders	[3]									(1,054)	(1,269)		(1,524)
Non-controlling Interest	[3]									(520)	(653)		(811)
Total dividends paid to common shareholders, LTIP unit holders, and non-controlling interest	[3],[4]									(83,460)	(86,449)		(92,067)
Undistributed (Distributed in excess of) earnings:
Common shareholders										(44,282)	(26,221)		(12,513)
LTIP unit holders										(569)	(408)		(219)
Non-controlling interest										(281)	(181)		(76)
Total undistributed (distributed in excess of) earnings attributable to common shareholders, LTIP unit holders, and non-controlling interest										$ (45,132)	$ (26,810)		$ (12,808)
Weighted average shares outstanding (basic and diluted):
Weighted average common shares outstanding										33,422,053	27,962,304		23,587,692
Weighted average participating LTIP units										431,640	413,105		398,789
Weighted average non-controlling interest units										212,000	212,000		212,000
Basic earnings per common share:
Distributed (in usd per share)										$ 2.45	$ 3.08		$ 3.83
Undistributed (Distributed in excess of) (in usd per share)										(1.32)	(0.99)		(0.55)
Basic earnings per common share (in usd per share)										1.13	2.09		3.28
Diluted earnings per common share:
Distributed (in usd per share)										2.45	3.08		3.83
Undistributed (Distributed in excess of) (in usd per share)										(1.32)	(0.99)		(0.55)
Diluted earnings per common share (in usd per share)										$ 1.13	$ 2.09		$ 3.28
Net Income (Loss) Attributable to Non-Participating Non-controlling Interest										$ 100	$ 300		$ 100

[1] Conformed to current period presentation.

[2] For the years ended December 31, 2015, 2014, and 2013, excludes net increase in equity resulting from operations of $0.1 million, $0.3 million, and $0.1 million, respectively attributable to joint venture partners, which have non-participating interests as described in Note 9.

[3] 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.

[4] For the years ended December 31, 2015, 2014, and 2013, dividends totaling $2.45, $3.08, and $3.83, respectively, per common share and convertible unit outstanding, were declared and paid.