Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 6,661	$ 6,195	$ 48,892		$ 26,537
Net Income (Loss) Attributable to Participating Non-controlling Interest	[1]	46	41	335		172
Net income (loss) attributable to parent and participating non-controlling interest		6,707	6,236	49,227		26,709
Dividends Paid:
Dividends	[2]	(12,651)	(14,758)	(38,152)	[3]	(44,336)	[3]
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ (5,944)	$ (8,522)	$ 11,075		$ (17,627)
Basic earnings per common share:
Distributed (in usd per share)		$ 0.41	$ 0.45	$ 1.23		$ 1.35
Undistributed (Distributed in excess of) (in usd per share)		(0.19)	(0.26)	0.35		(0.54)
Basic earnings per common share (in usd per share)		0.22	0.19	1.58		0.81
Diluted earnings per common share:
Distributed (in usd per share)		0.41	0.45	1.23		1.35
Undistributed (Distributed in excess of) (in usd per share)		(0.19)	(0.26)	0.35		(0.54)
Diluted earnings per common share (in usd per share)		$ 0.22	$ 0.19	$ 1.58		$ 0.81
Net Income (Loss) Attributable to Non-Participating Non-controlling Interest		$ 800	$ 400	$ 1,800		$ 1,100
Common Stock [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		6,553	6,106	48,111		26,155
Dividends Paid:
Dividends	[2]	(12,362)	(14,451)	(37,285)		(43,416)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ (5,809)	$ (8,345)	$ 10,826		$ (17,261)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted		30,151,753	32,095,675	30,392,390		32,153,366
Long-Term Incentive Plan Units [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 108	$ 89	$ 781		$ 382
Dividends Paid:
Dividends	[2]	(202)	(211)	(606)		(634)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ (94)	$ (122)	$ 175		$ (252)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted		495,212	471,057	493,257		470,050
Noncontrolling Interest [Member]
Dividends Paid:
Dividends	[2]	$ (87)	$ (96)	$ (261)		$ (286)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ (41)	$ (55)	$ 74		$ (114)
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 nine-month periods ended September 30, 2018 and 2017, excludes net increase (decrease) in equity resulting from operations of $1.8 million and $1.1 million, respectively attributable to joint venture partners, which have non-participating interests as described in Note 11.

[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, 2018 and 2017, dividends totaling $1.23 and $1.35, respectively, per common share and convertible unit outstanding, were declared.