Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ (2,217)	$ 6,661	$ 21,193	$ 21,039	$ 7,444	$ 6,195	$ 5,064	$ 15,278	$ 46,676	$ 33,981
Net Income (Loss) Attributable to Participating Non-controlling Interest	[1]									319	221
Net income (loss) attributable to parent and participating non-controlling interest										46,995	34,202
Dividends Paid:
Dividends	[2]									(50,736)	(57,636)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings										$ (3,741)	$ (23,434)
Basic earnings per common share:
Distributed (in usd per share)										$ 1.64	$ 1.76
Undistributed (Distributed in excess of) (in usd per share)										(0.12)	(0.72)
Basic earnings per common share (in usd per share)										1.52	1.04
Diluted earnings per common share:
Distributed (in usd per share)										1.64	1.76
Undistributed (Distributed in excess of) (in usd per share)										(0.12)	(0.72)
Diluted earnings per common share (in usd per share)										$ 1.52	$ 1.04
Net Income (Loss) Attributable to Non-Participating Non-controlling Interest										$ 2,900	$ 1,800
Common Stock [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent										45,922	33,487
Dividends Paid:
Dividends										(49,576)	(56,434)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings										$ (3,653)	$ (22,947)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted										30,297,401	32,062,091
Long-Term Incentive Plan Units [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent										$ 753	$ 494
Dividends Paid:
Dividends										(812)	(829)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings										$ (59)	$ (335)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted										496,962	472,527
Noncontrolling Interest [Member]
Dividends Paid:
Dividends										$ (348)	$ (373)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings										$ (29)	$ (152)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted										212,000	212,000

[1] For the years ended December 31, 2018 and 2017, excludes net increase (decrease) in equity resulting from operations of $2.9 million and $1.8 million, respectively attributable to joint venture partners, which have non-participating interests as described in Note 11.

[2] For the years ended December 31, 2018 and 2017, dividends totaling $1.64 and $1.76, respectively, per common share and convertible unit outstanding, were declared.