Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 6,195	$ 516	$ 26,537		$ (17,699)
Net Income (Loss) Attributable to Participating Non-controlling Interest	[1]	41	3	172		(112)
Net income (loss) attributable to parent and participating non-controlling interest		6,236	519	26,709		(17,811)
Dividends Paid:
Dividends	[2]	(14,758)	(16,640)	(44,336)	[3]	(50,256)	[3]
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ (8,522)	$ (16,121)	$ (17,627)		$ (68,067)
Basic earnings per common share:
Distributed (in usd per share)		$ 0.45	$ 0.50	$ 1.35		$ 1.50
Undistributed (Distributed in excess of) (in usd per share)		(0.26)	(0.48)	(0.54)		(2.03)
Basic earnings per common share (in usd per share)		0.19	0.02	0.81		(0.53)
Diluted earnings per common share:
Distributed (in usd per share)		0.45	0.50	1.35		1.50
Undistributed (Distributed in excess of) (in usd per share)		(0.26)	(0.48)	(0.54)		(2.03)
Diluted earnings per common share (in usd per share)		$ 0.19	$ 0.02	$ 0.81		$ (0.53)
Net Income (Loss) Attributable to Non-Participating Non-controlling Interest		$ 400	$ 31	$ 1,100		$ 200
Common Stock [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		6,106	509	26,155		(17,460)
Dividends Paid:
Dividends	[2]	(14,451)	(16,309)	(43,416)		(49,263)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ (8,345)	$ (15,800)	$ (17,261)		$ (66,723)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted		32,095,675	32,643,154	32,153,366		32,854,979
Long-Term Incentive Plan Units [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 89	$ 7	$ 382		$ (239)
Dividends Paid:
Dividends	[2]	(211)	(225)	(634)		(675)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ (122)	$ (218)	$ (252)		$ (914)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted		471,057	451,125	470,050		450,336
Noncontrolling Interest [Member]
Dividends Paid:
Dividends	[2]	$ (96)	$ (106)	$ (286)		$ (318)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ (55)	$ (103)	$ (114)		$ (430)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted		212,000	212,000	212,000		212,000

[1] For the three month periods ended September 30, 2017 and 2016, excludes net increase in equity resulting from operations of $0.4 million and $31 thousand, respectively, attributable to joint venture partners, which have non-participating interests as described in Note 9. For the nine month periods ended September 30, 2017 and 2016, excludes net increase in equity resulting from operations of $1.1 million and $0.2 million, respectively attributable to joint venture partners, which have non-participating interests as described in Note 9.

[2] 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.

[3] For the nine month periods ended September 30, 2017 and 2016, dividends totaling $1.35 and $1.50 respectively, per common share and convertible unit outstanding, were declared and paid.