Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 21,039	$ 15,278
Net Income (Loss) Attributable to Participating Non-controlling Interest	[1]	142	99
Net income (loss) attributable to parent and participating non-controlling interest		21,181	15,377
Dividends Paid:
Dividends	[2],[3]	(12,850)	(14,821)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ 8,331	$ 556
Basic earnings per common share:
Distributed (in usd per share)		$ 0.41	$ 0.45
Undistributed (Distributed in excess of) (in usd per share)		0.26	0.02
Basic earnings per common share (in usd per share)		0.67	0.47
Diluted earnings per common share:
Distributed (in usd per share)		0.41	0.45
Undistributed (Distributed in excess of) (in usd per share)		0.26	0.02
Diluted earnings per common share (in usd per share)		$ 0.67	$ 0.47
Net Income (Loss) Attributable to Non-Participating Non-controlling Interest		$ 100	$ 400
Common Stock [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		20,709	15,059
Dividends Paid:
Dividends	[3]	(12,562)	(14,515)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ 8,147	$ 544
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted		30,830,615	32,248,613
Long-Term Incentive Plan Units [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 330	$ 219
Dividends Paid:
Dividends	[3]	(201)	(211)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ 129	$ 8
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted		491,638	469,539
Noncontrolling Interest [Member]
Dividends Paid:
Dividends	[3]	$ (87)	$ (95)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ 55	$ 4
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted		212,000	212,000

[1] For the three month periods ended March 31, 2018 and 2017, excludes net increase in equity resulting from operations of $0.1 million and $0.4 million, respectively attributable to joint venture partners, which have non-participating interests as described in Note 11.

[2] For the three month periods ended March 31, 2018 and 2017, dividends totaling $0.41 and $0.45, 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.