CONSOLIDATED STATEMENTS OF EQUITY - USD ($)	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, 2017	$ 755,160,000	$ 396,000	$ 856,992,000	$ 1,100,783,000	$ (1,203,011,000)	$ 3,488,000	$ 758,648,000
Balance (in shares) at Dec. 31, 2017		39,570,000
Equity activity
Common stock cash distributions	(22,578,000)				(22,578,000)		(22,578,000)
Issuance of restricted stock (in shares)		82,000
Stock option exercises	123,000		123,000				123,000
Stock option exercises (in shares)		5,000
Stock-based compensation expense	1,376,000		1,376,000				1,376,000
Net income	20,359,000			20,359,000			20,359,000
Other	(1,065,000)		(1,065,000)				(1,065,000)
Other (in shares)		(28,000)
Balance at end of period at Mar. 31, 2018	753,375,000	$ 396,000	857,426,000	1,121,142,000	(1,225,589,000)	3,488,000	756,863,000
Balance (in shares) at Mar. 31, 2018		39,629,000
Balance at beginning of period at Dec. 31, 2017	755,160,000	$ 396,000	856,992,000	1,100,783,000	(1,203,011,000)	3,488,000	$ 758,648,000
Balance (in shares) at Dec. 31, 2017		39,570,000
Equity activity
Stock option exercises (in shares)							5,000
Net income							$ 124,232,000
Balance at end of period at Sep. 30, 2018	815,842,000	$ 397,000	861,226,000	1,224,998,000	(1,270,779,000)	7,451,000	823,293,000
Balance (in shares) at Sep. 30, 2018		39,657,000
Balance at beginning of period at Mar. 31, 2018	753,375,000	$ 396,000	857,426,000	1,121,142,000	(1,225,589,000)	3,488,000	756,863,000
Balance (in shares) at Mar. 31, 2018		39,629,000
Equity activity
Common stock cash distributions	(22,590,000)				(22,590,000)		(22,590,000)
Issuance of restricted stock	(8,000)		(8,000)				(8,000)
Issuance of restricted stock (in shares)		9,000
Stock-based compensation expense	1,521,000		1,521,000				1,521,000
Net income	68,936,000			68,936,000			68,936,000
Non-controlling interests contribution						1,081,000	1,081,000
Other	(107,000)		(107,000)				(107,000)
Other (in shares)		(3,000)
Balance at end of period at Jun. 30, 2018	801,127,000	$ 396,000	858,832,000	1,190,078,000	(1,248,179,000)	4,569,000	805,696,000
Balance (in shares) at Jun. 30, 2018		39,635,000
Equity activity
Common stock cash distributions	(22,600,000)				(22,600,000)		(22,600,000)
Proceeds from common stock issued, net of issuance costs	929,000	$ 1,000	928,000				929,000
Proceeds from common stock issued, net of issuance costs (in shares)		22,000
Stock-based compensation expense	1,487,000		1,487,000				1,487,000
Net income	34,920,000			34,920,000		17,000	34,937,000
Non-controlling interests contribution						2,882,000	2,882,000
Non-controlling interest distributions						(17,000)	(17,000)
Other	(21,000)		(21,000)				(21,000)
Balance at end of period at Sep. 30, 2018	815,842,000	$ 397,000	861,226,000	1,224,998,000	(1,270,779,000)	7,451,000	823,293,000
Balance (in shares) at Sep. 30, 2018		39,657,000
Equity activity
Common stock cash distributions	(22,604,000)				(22,604,000)		(22,604,000)
Stock-based compensation expense	1,486,000		1,486,000				1,486,000
Net income	30,766,000			30,766,000		78,000	30,844,000
Non-controlling interest distributions						(48,000)	(48,000)
Balance at end of period at Dec. 31, 2018	825,490,000	$ 397,000	862,712,000	1,255,764,000	(1,293,383,000)	7,481,000	832,971,000
Balance (in shares) at Dec. 31, 2018		39,657,000
Equity activity
Cumulative effect of the adoption of the ASC 842	(42,808,000)			(42,808,000)			(42,808,000)
As Adjusted Balance at January 1, 2019	782,682,000	$ 397,000	862,712,000	1,212,956,000	(1,293,383,000)	7,481,000	790,163,000
Common stock cash distributions	(22,931,000)				(22,931,000)		(22,931,000)
Common stock cash distributions (in shares)		48,000
Issuance of restricted stock	(1,000)		(1,000)				(1,000)
Issuance of restricted stock (in shares)		78,000
Stock-based compensation expense	1,689,000		1,689,000				1,689,000
Net income	20,346,000			20,346,000		81,000	20,427,000
Non-controlling interests contribution						919,000	919,000
Non-controlling interest distributions						(89,000)	(89,000)
Other	(2,024,000)		(2,024,000)				(2,024,000)
Other (in shares)		(44,000)
Balance at end of period at Mar. 31, 2019	779,761,000	$ 397,000	862,376,000	1,233,302,000	(1,316,314,000)	8,392,000	788,153,000
Balance (in shares) at Mar. 31, 2019		39,739,000
Balance at beginning of period at Dec. 31, 2018	825,490,000	$ 397,000	862,712,000	1,255,764,000	(1,293,383,000)	7,481,000	$ 832,971,000
Balance (in shares) at Dec. 31, 2018		39,657,000
Equity activity
Stock option exercises (in shares)							5,000
Net income							$ 68,241,000
Balance at end of period at Sep. 30, 2019	785,434,000	$ 398,000	865,721,000	1,280,940,000	(1,361,625,000)	8,460,000	793,894,000
Balance (in shares) at Sep. 30, 2019		39,752,000
Balance at beginning of period at Mar. 31, 2019	779,761,000	$ 397,000	862,376,000	1,233,302,000	(1,316,314,000)	8,392,000	788,153,000
Balance (in shares) at Mar. 31, 2019		39,739,000
Equity activity
Common stock cash distributions	(22,653,000)				(22,653,000)		(22,653,000)
Issuance of restricted stock	(6,000)		(6,000)				(6,000)
Issuance of restricted stock (in shares)		8,000
Stock-based compensation expense	1,623,000		1,623,000				1,623,000
Net income	20,446,000			20,446,000		88,000	20,534,000
Non-controlling interests contribution						46,000	46,000
Non-controlling interest distributions						(87,000)	(87,000)
Balance at end of period at Jun. 30, 2019	779,171,000	$ 397,000	863,993,000	1,253,748,000	(1,338,967,000)	8,439,000	787,610,000
Balance (in shares) at Jun. 30, 2019		39,747,000
Equity activity
Common stock cash distributions	(22,658,000)				(22,658,000)		(22,658,000)
Stock option exercises	123,000	$ 1,000	122,000				123,000
Stock option exercises (in shares)		5,000
Stock-based compensation expense	1,626,000		1,626,000				1,626,000
Net income	27,192,000			27,192,000		88,000	27,280,000
Non-controlling interest distributions						(67,000)	(67,000)
Other	(20,000)		(20,000)				(20,000)
Balance at end of period at Sep. 30, 2019	$ 785,434,000	$ 398,000	$ 865,721,000	$ 1,280,940,000	$ (1,361,625,000)	$ 8,460,000	$ 793,894,000
Balance (in shares) at Sep. 30, 2019		39,752,000