Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 7,444	$ 6,195	$ 5,064	$ 15,278	$ 1,692	$ 516	$ 4,985	$ (23,200)	$ 33,981	$ (16,007)	$ 38,089
Net Income (Loss) Attributable to Participating Non-controlling Interest	[1]									221	(101)	239
Net income (loss) attributable to parent and participating non-controlling interest										34,202	(16,108)	38,328
Dividends Paid:
Dividends	[2],[3]									(57,636)	(65,148)	(83,460)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings										$ (23,434)	$ (81,256)	$ (45,132)
Basic earnings per common share:
Distributed (in usd per share)										$ 1.76	$ 1.95	$ 2.45
Undistributed (Distributed in excess of) (in usd per share)										(0.72)	(2.43)	(1.32)
Basic earnings per common share (in usd per share)										1.04	(0.48)	1.13
Diluted earnings per common share:
Distributed (in usd per share)										1.76	1.95	2.45
Undistributed (Distributed in excess of) (in usd per share)										(0.72)	(2.43)	(1.32)
Diluted earnings per common share (in usd per share)										$ 1.04	$ (0.48)	$ 1.13
Net Income (Loss) Attributable to Non-Participating Non-controlling Interest										$ 1,800	$ 400	$ 100
Common Stock [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent										33,487	(15,789)	37,604
Dividends Paid:
Dividends	[3]									(56,434)	(63,855)	(81,886)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings										$ (22,947)	$ (79,644)	$ (44,282)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted										32,062,091	32,758,050	33,422,053
Long-Term Incentive Plan Units [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent										$ 494	$ (218)	$ 485
Dividends Paid:
Dividends	[3]									(829)	(880)	(1,054)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings										$ (335)	$ (1,098)	$ (569)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted										472,527	452,436	431,640
Noncontrolling Interest [Member]
Dividends Paid:
Dividends	[3]									$ (373)	$ (413)	$ (520)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings										$ (152)	$ (514)	$ (281)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted										212,000	212,000	212,000

[1] For the years ended December 31, 2017, 2016, and 2015, excludes net increase in equity resulting from operations of $1.8 million, $0.4 million, and $0.1 million, respectively attributable to joint venture partners, which have non-participating interests as described in Note 11.

[2] For the years ended December 31, 2017, 2016, and 2015, dividends totaling $1.76, $1.95, and $2.45, respectively, per common share and convertible unit outstanding, were declared and paid.

[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.