Consolidated Statements of Changes In Equity - USD ($) $ in Thousands	Total	Common Stock	Paid-in Capital	Redeemable Perpetual Preferred Stock	Distributions in Excess of Accumulated Earnings	Accumulated Other Comprehensive Income (Loss)	Non- controlling interests – Common OP Units
Balance at Dec. 31, 2017	$ 1,100,042	$ 1,766	$ 1,241,226		$ (211,980)	$ 942	$ 68,088
Increase (Decrease) in Stockholders' Equity
Exchange of Common OP Units for common stock			80				(80)
Issuance of common stock through employee stock purchase plan	503		503
Compensation expenses related to restricted stock and stock options	1,800		1,800
Adjustment for Common OP Unitholders in the Operating Partnership			782				(782)
Adjustment for fair market value of swap	1,873					1,873
Consolidated net income	64,177				60,222		3,955
Distributions	(52,010)				(48,805)		(3,205)
Other	(60)		(60)
Balance at Mar. 31, 2018	1,101,139	1,766	1,244,331		(215,749)	2,815	67,976
Balance at Dec. 31, 2017	1,100,042	1,766	1,241,226		(211,980)	942	68,088
Increase (Decrease) in Stockholders' Equity
Adjustment for fair market value of swap	3,017
Balance at Sep. 30, 2018	1,190,517	1,790	1,324,753	$ 0	(211,743)	3,959	71,758
Balance at Mar. 31, 2018	1,101,139	1,766	1,244,331		(215,749)	2,815	67,976
Increase (Decrease) in Stockholders' Equity
Exchange of Common OP Units for common stock		2	80				(82)
Issuance of common stock through employee stock purchase plan	343		343
Compensation expenses related to restricted stock and stock options	2,741		2,741
Adjustment for Common OP Unitholders in the Operating Partnership			(57)				57
Adjustment for fair market value of swap	764					764
Consolidated net income	49,169			8	46,137		3,024
Distributions	(52,050)			(8)	(48,841)		(3,201)
Other	(275)		(275)
Balance at Jun. 30, 2018	1,101,831	1,768	1,247,163	0	(218,453)	3,579	67,774
Increase (Decrease) in Stockholders' Equity
Exchange of Common OP Units for common stock		2	857				(859)
Issuance of common stock through employee stock purchase plan	765		765
Issuance of Common Stock	78,755	20	78,735
Compensation expenses related to restricted stock and stock options	2,746		2,746
Adjustment for Common OP Unitholders in the Operating Partnership			(4,414)				4,414
Adjustment for fair market value of swap	380					380
Consolidated net income	59,660				56,070		3,590
Distributions	(52,521)				(49,360)		(3,161)
Other	(1,099)		(1,099)
Balance at Sep. 30, 2018	1,190,517	1,790	1,324,753	0	(211,743)	3,959	71,758
Balance at Dec. 31, 2018	1,193,344	1,792	1,328,495	0	(211,034)	2,299	71,792
Increase (Decrease) in Stockholders' Equity
Exchange of Common OP Units for common stock			66				(66)
Issuance of common stock through exercise of options	53		53
Issuance of common stock through employee stock purchase plan	652		652
Compensation expenses related to restricted stock and stock options	2,420		2,420
Repurchase of common stock or Common OP Units	(53)		(53)
Adjustment for Common OP Unitholders in the Operating Partnership			(56)				56
Adjustment for fair market value of swap	(931)					(931)
Consolidated net income	120,535				113,309		7,226
Distributions	(58,639)				(55,123)		(3,516)
Other	(63)		(63)
Balance at Mar. 31, 2019	1,257,318	1,792	1,331,514	0	(152,848)	1,368	75,492
Balance at Dec. 31, 2018	1,193,344	1,792	1,328,495	0	(211,034)	2,299	71,792
Increase (Decrease) in Stockholders' Equity
Adjustment for fair market value of swap	(2,798)
Balance at Sep. 30, 2019	1,319,793	1,812	1,399,951	0	(153,505)	(499)	72,034
Balance at Mar. 31, 2019	1,257,318	1,792	1,331,514	0	(152,848)	1,368	75,492
Increase (Decrease) in Stockholders' Equity
Exchange of Common OP Units for common stock		10	6,425				(6,435)
Issuance of common stock through employee stock purchase plan	587		587
Issuance of Common Stock	59,319	10	59,309
Compensation expenses related to restricted stock and stock options	2,625		2,625
Adjustment for Common OP Unitholders in the Operating Partnership			(2,883)				2,883
Adjustment for fair market value of swap	(1,610)					(1,610)	0
Consolidated net income	49,085				46,401		2,676
Distributions	(58,980)			(8)	(55,757)		(3,215)
Other	(870)		(870)
Balance at Jun. 30, 2019	1,307,474	1,812	1,396,707	0	(162,204)	(242)	71,401
Increase (Decrease) in Stockholders' Equity
Exchange of Common OP Units for common stock			33				(33)
Issuance of common stock through employee stock purchase plan	698		698
Compensation expenses related to restricted stock and stock options	2,734		2,734
Adjustment for Common OP Unitholders in the Operating Partnership			(165)				165
Adjustment for fair market value of swap	(257)					(257)
Consolidated net income	68,176				64,461		3,715
Distributions	(58,976)				(55,762)		(3,214)
Other	(56)		(56)
Balance at Sep. 30, 2019	$ 1,319,793	$ 1,812	$ 1,399,951	$ 0	$ (153,505)	$ (499)	$ 72,034