Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 15,278	$ (23,200)
Net Income (Loss) Attributable to Participating Non-controlling Interest	[1]	99	(147)
Net income (loss) attributable to parent and participating non-controlling interest		15,377	(23,347)
Dividends Paid:
Dividends	[2],[3]	(14,821)	(16,872)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ 556	$ (40,219)
Basic earnings per common share:
Distributed (in usd per share)		$ 0.45	$ 0.50
Undistributed (Distributed in excess of) (in usd per share)		0.02	(1.19)
Basic earnings per common share (in usd per share)		0.47	(0.69)
Diluted earnings per common share:
Distributed (in usd per share)		0.45	0.50
Undistributed (Distributed in excess of) (in usd per share)		0.02	(1.19)
Diluted earnings per common share (in usd per share)		$ 0.47	$ (0.69)
Net Income (Loss) Attributable to Non-Participating Non-controlling Interest		$ 400	$ 200
Common Stock [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		15,059	(22,889)
Dividends Paid:
Dividends	[3]	(14,515)	(16,541)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ 544	$ (39,430)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted		32,248,613	33,083,682
Long-Term Incentive Plan Units [Member]
Net increase (decrease) in shareholders' equity resulting from operations
Net Income (Loss) Attributable to Parent		$ 219	$ (311)
Dividends Paid:
Dividends	[3]	(211)	(225)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ 8	$ (536)
Weighted average shares outstanding (basic and diluted):
Weighted Average Number of Shares Outstanding, Basic and Diluted		469,539	449,938
Noncontrolling Interest [Member]
Dividends Paid:
Dividends	[3]	$ (95)	$ (106)
Undistributed (Distributed in excess of) earnings:
Total undistributed (distributed in excess of) earnings		$ 4	$ (253)
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, 2017 and 2016, excludes net increase in equity resulting from operations of $0.4 million and $0.2 million, respectively attributable to joint venture partners, which have non-participating interests as described in Note 9.

[2] For the three month periods ended March 31, 2017 and 2016, dividends totaling $0.45 and $0.50 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.