Earnings Per Share (Details) (USD $)	3 Months Ended									12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014		Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent	$ 2,637,000	$ 12,948,000	[1]	$ 20,947,000	$ 22,635,000	$ 14,862,000	$ 11,727,000	$ 11,600,000	$ 40,335,000	$ 59,167,000		$ 78,524,000		$ 97,146,000
Net Income (Loss) Attributable to Participating Non-controlling Interest										472,000	[2]	735,000	[2]	0	[2]
Net income (loss) attributable to parent and participating non-controlling interest										59,639,000		79,259,000		97,146,000
Net increase in shareholders' equity resulting from operations– common shares										58,306,000		77,219,000		95,064,000
Net increase in shareholders' equity resulting from operations– LTIP units										861,000		1,305,000		2,082,000
Dividends Paid(2):
Common shareholders										(84,527,000)	[3]	(89,732,000)	[3]	(46,433,000)	[3]
LTIP unit holders										(1,269,000)	[3]	(1,524,000)	[3]	(979,000)	[3]
Non-controlling Interest										(653,000)	[3]	(811,000)	[3]	0	[3]
Total dividends paid to common shareholders, LTIP unit holders, and non-controlling interest										(86,449,000)	[3],[4]	(92,067,000)	[3],[4]	(47,412,000)	[3],[4]
Undistributed (Distributed in excess of) earnings:
Common shareholders										(26,221,000)		(12,513,000)		48,631,000
LTIP Unit holders										(408,000)		(219,000)		1,103,000
Non-controlling interest										(181,000)		(76,000)		0
Total undistributed (distributed in excess of) earnings attributable to common shareholders, LTIP unit holders, and non-controlling interest										(26,810,000)		(12,808,000)		49,734,000
Weighted average shares outstanding (basic and diluted):
Weighted average common shares outstanding										27,962,304		23,587,692		17,897,996
Weighted average participating LTIP units										413,105		398,789		391,944
Weighted average non-controlling interest units										212,000		212,000		0
Basic earnings per common share:
Distributed (in usd per share)										$ 3.08		$ 3.83		$ 2.50
Undistributed (in usd per share)										$ (0.99)		$ (0.55)		$ 2.81
Basic earnings per common share (in usd per share)										$ 2.09		$ 3.28		$ 5.31
Diluted earnings per common share:
Distributed (in usd per share)										$ 3.08		$ 3.83		$ 2.50
Undistributed (in usd per share)										$ (0.99)		$ (0.55)		$ 2.81
Diluted earnings per common share (in usd per share)										$ 2.09		$ 3.28		$ 5.31
Net Income (Loss) Attributable to Non-Participating Non-controlling Interest										$ 300,000		$ 100,000

[1] Prior period conformed to current period presentation.

[2] For the years ended December 31, 2014 and 2013, excludes net increase in equity resulting from operations of $0.3 million and $0.1 million, respectively attributable to the joint venture partner, which has 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, 2014, 2013, and 2012, dividends totaling $3.08, $3.83, and $2.50, respectively, per common share and convertible unit outstanding, were declared and paid.