CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Thousands	Parent	Common Stock	Capital in Excess of Par Value	Cumulative Net Income	Cumulative Distributions	Non-controlling Interests	Total
Balance at beginning of period at Dec. 31, 2016	$ 740,048	$ 392	$ 839,005	$ 1,013,443	$ (1,112,792)		$ 740,048
Balance (in shares) at Dec. 31, 2016		39,221,000
Equity activity
Common Stock cash distributions	(22,552)				(22,552)		(22,552)
Proceeds from common stock issued, net of issuance costs	14,529	$ 3	14,526				14,529
Proceeds from common stock issued, net of issuance costs (in shares)		313,000
Issuance of restricted stock	(6)	$ 1	(7)				(6)
Issuance of restricted stock (in shares)		75,000
Stock-based compensation expense	1,259		1,259				1,259
Net income	21,513			21,513			21,513
Other	(1,651)		(1,651)				(1,651)
Other (in shares)		(36,000)
Balance at end of period at Mar. 31, 2017	753,140	$ 396	853,132	1,034,956	(1,135,344)		753,140
Balance (in shares) at Mar. 31, 2017		39,573,000
Balance at beginning of period at Dec. 31, 2016	740,048	$ 392	839,005	1,013,443	(1,112,792)		$ 740,048
Balance (in shares) at Dec. 31, 2016		39,221,000
Equity activity
Stock option exercises (in shares)							8,334
Net income							$ 67,506
Balance at end of period at Sep. 30, 2017	756,635	$ 396	855,746	1,080,949	(1,180,456)		756,635
Balance (in shares) at Sep. 30, 2017		39,571,000
Balance at beginning of period at Mar. 31, 2017	753,140	$ 396	853,132	1,034,956	(1,135,344)		753,140
Balance (in shares) at Mar. 31, 2017		39,573,000
Equity activity
Common Stock cash distributions	(22,558)				(22,558)		(22,558)
Issuance of restricted stock (in shares)		8,000
Cancelation of restricted stock (in shares)		(14,000)
Stock option exercises	79		79				79
Stock option exercises (in shares)		3,000
Stock-based compensation expense	1,425		1,425				1,425
Net income	25,377			25,377			25,377
Other	(296)		(296)				(296)
Other (in shares)		(6,000)
Balance at end of period at Jun. 30, 2017	757,167	$ 396	854,340	1,060,333	(1,157,902)		757,167
Balance (in shares) at Jun. 30, 2017		39,564,000
Equity activity
Common Stock cash distributions	(22,554)				(22,554)		(22,554)
Issuance of restricted stock (in shares)		3,000
Cancelation of restricted stock (in shares)		(1,000)
Stock option exercises	123		123				123
Stock option exercises (in shares)		5,000
Stock-based compensation expense	1,283		1,283				1,283
Net income	20,616			20,616			20,616
Balance at end of period at Sep. 30, 2017	756,635	$ 396	855,746	1,080,949	(1,180,456)		756,635
Balance (in shares) at Sep. 30, 2017		39,571,000
Equity activity
Common Stock cash distributions	(22,555)				(22,555)		(22,555)
Issuance of restricted stock	(14)		(14)				(14)
Stock-based compensation expense	1,282		1,282				1,282
Net income	19,834			19,834			19,834
Non-controlling interests contribution						$ 3,488	3,488
Other	(22)		(22)				(22)
Other (in shares)		(1,000)
Balance at end of period at Dec. 31, 2017	755,160	$ 396	856,992	1,100,783	(1,203,011)	3,488	758,648
Balance (in shares) at Dec. 31, 2017		39,570,000
Equity activity
Common Stock cash distributions	(22,578)				(22,578)		(22,578)
Issuance of restricted stock (in shares)		82,000
Stock option exercises	123		123				123
Stock option exercises (in shares)		5,000
Stock-based compensation expense	1,376		1,376				1,376
Net income	20,359			20,359			20,359
Other	(1,065)		(1,065)				(1,065)
Other (in shares)		(28,000)
Balance at end of period at Mar. 31, 2018	753,375	$ 396	857,426	1,121,142	(1,225,589)	3,488	756,863
Balance (in shares) at Mar. 31, 2018		39,629,000
Balance at beginning of period at Dec. 31, 2017	755,160	$ 396	856,992	1,100,783	(1,203,011)	3,488	$ 758,648
Balance (in shares) at Dec. 31, 2017		39,570,000
Equity activity
Stock option exercises (in shares)							5,000
Net income							$ 124,232
Balance at end of period at Sep. 30, 2018	815,842	$ 397	861,226	1,224,998	(1,270,779)	7,451	823,293
Balance (in shares) at Sep. 30, 2018		39,657,000
Balance at beginning of period at Mar. 31, 2018	753,375	$ 396	857,426	1,121,142	(1,225,589)	3,488	756,863
Balance (in shares) at Mar. 31, 2018		39,629,000
Equity activity
Common Stock cash distributions	(22,590)				(22,590)		(22,590)
Issuance of restricted stock	(8)		(8)				(8)
Issuance of restricted stock (in shares)		9,000
Stock-based compensation expense	1,521		1,521				1,521
Net income	68,936			68,936			68,936
Non-controlling interests contribution						1,081	1,081
Other	(107)		(107)				(107)
Other (in shares)		(3,000)
Balance at end of period at Jun. 30, 2018	801,127	$ 396	858,832	1,190,078	(1,248,179)	4,569	805,696
Balance (in shares) at Jun. 30, 2018		39,635,000
Equity activity
Common Stock cash distributions	(22,600)				(22,600)		(22,600)
Proceeds from common stock issued, net of issuance costs	929	$ 1	928				929
Proceeds from common stock issued, net of issuance costs (in shares)		22,000
Stock-based compensation expense	1,487		1,487				1,487
Net income	34,920			34,920		17	34,937
Non-controlling interests contribution						2,882	2,882
Non-controlling interest distributions						17	17
Other	(21)		(21)				(21)
Balance at end of period at Sep. 30, 2018	$ 815,842	$ 397	$ 861,226	$ 1,224,998	$ (1,270,779)	$ 7,451	$ 823,293
Balance (in shares) at Sep. 30, 2018		39,657,000