Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 21,193	$ 5,064	$ 42,232		$ 20,342
Net Income (Loss) Attributable to Participating Non-controlling Interest	[1]	146	33	289		132
Net income (loss) attributable to parent and participating non-controlling interest		21,339	5,097	42,521		20,474
Dividends Paid:
Dividends	[2]	(12,650)	(14,757)	(25,501)	[3]	(29,579)	[3]
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ 8,689	$ (9,660)	$ 17,020		$ (9,105)
Basic earnings per common share:
Distributed (in usd per share)		$ 0.41	$ 0.45	$ 0.82		$ 0.90
Undistributed (Distributed in excess of) (in usd per share)		0.28	(0.29)	0.54		(0.28)
Basic earnings per common share (in usd per share)		0.69	0.16	1.36		0.62
Diluted earnings per common share:
Distributed (in usd per share)		0.41	0.45	0.82		0.90
Undistributed (Distributed in excess of) (in usd per share)		0.28	(0.29)	0.54		(0.28)
Diluted earnings per common share (in usd per share)		$ 0.69	$ 0.16	$ 1.36		$ 0.62
Net Income (Loss) Attributable to Non-Participating Non-controlling Interest		$ 800	$ 300	$ 1,000		$ 700
Common Stock [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		20,853	4,991	41,561		20,049
Dividends Paid:
Dividends	[2]	(12,361)	(14,451)	(24,923)		(28,965)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ 8,492	$ (9,460)	$ 16,638		$ (8,916)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted		30,202,262	32,117,492	30,514,702		32,182,690
Long-Term Incentive Plan Units [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 340	$ 73	$ 671		$ 293
Dividends Paid:
Dividends	[2]	(202)	(211)	(404)		(423)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ 138	$ (138)	$ 267		$ (130)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted		492,882	469,539	492,263		469,539
Noncontrolling Interest [Member]
Dividends Paid:
Dividends	[2]	$ (87)	$ (95)	$ (174)		$ (191)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ 59	$ (62)	$ 115		$ (59)
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 six-month periods ended June 30, 2018 and 2017, excludes net increase (decrease) in equity resulting from operations of $1.0 million and $0.7 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 six-month periods ended June 30, 2018 and 2017, dividends totaling $0.82 and $0.90, respectively, per common share and convertible unit outstanding, were declared and paid.