Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 1,692	$ 516	$ 4,985	$ (23,200)	$ 1,780	$ 3,896	$ 13,152	$ 19,261	$ (16,007)	$ 38,089	$ 59,167
Net Income (Loss) Attributable to Participating Non-controlling Interest	[1]									(101)	239	472
Net income (loss) attributable to parent and participating non-controlling interest										(16,108)	38,328	59,639
Dividends Paid:
Dividends	[2],[3]									(65,148)	(83,460)	(86,449)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings										$ (81,256)	$ (45,132)	$ (26,810)
Basic earnings per common share:
Distributed (in usd per share)										$ 1.95	$ 2.45	$ 3.08
Undistributed (Distributed in excess of) (in usd per share)										(2.43)	(1.32)	(0.99)
Basic earnings per common share (in usd per share)										(0.48)	1.13	2.09
Diluted earnings per common share:
Distributed (in usd per share)										1.95	2.45	3.08
Undistributed (Distributed in excess of) (in usd per share)										(2.43)	(1.32)	(0.99)
Diluted earnings per common share (in usd per share)										$ (0.48)	$ 1.13	$ 2.09
Net Income (Loss) Attributable to Non-Participating Non-controlling Interest										$ 400	$ 100	$ 300
Common Stock [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent										(15,789)	37,604	58,306
Dividends Paid:
Dividends	[3]									(63,855)	(81,886)	(84,527)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings										$ (79,644)	$ (44,282)	$ (26,221)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted										32,758,050	33,422,053	27,962,304
Long-Term Incentive Plan Units [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent										$ (218)	$ 485	$ 861
Dividends Paid:
Dividends	[3]									(880)	(1,054)	(1,269)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings										$ (1,098)	$ (569)	$ (408)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted										452,436	431,640	413,105
Noncontrolling Interest [Member]
Dividends Paid:
Dividends	[3]									$ (413)	$ (520)	$ (653)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings										$ (514)	$ (281)	$ (181)
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, 2016, 2015, and 2014, excludes net increase in equity resulting from operations of $0.4 million, $0.1 million and $0.3 million, respectively attributable to joint venture partners, which have non-participating interests as described in Note 9.

[2] For the years ended December 31, 2016, 2015, and 2014, dividends totaling $1.95, $2.45, and $3.08, 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.