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 Loss/(Income)	Non- controlling interests – Common OP Units	Preferred Stock	Preferred Stock Paid-in Capital	Preferred Stock Redeemable Perpetual Preferred Stock
Balance at Dec. 31, 2016	$ 1,081,847	$ 854	$ 1,103,048	$ 136,144	$ (231,276)	$ (227)	$ 73,304
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exchange of Common OP Units for Common Stock	0	12	15,339				(15,351)
Issuance of Common Stock through employee stock purchase plan	403		403
Compensation expenses related to restricted stock and stock options	1,755		1,755
Adjustment for Common OP Unitholders in the Operating Partnership	0		(2,885)				2,885
Adjustment for fair market value of swap	226					226
Consolidated net income	63,075			2,297	56,888		3,890
Distributions	(47,527)			(2,297)	(42,335)		(2,895)
Other	(33)		(32)		(1)
Balance at Mar. 31, 2017	1,099,746	866	1,117,628	136,144	(216,724)	(1)	61,833
Balance at Dec. 31, 2016	1,081,847	854	1,103,048	136,144	(231,276)	(227)	73,304
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment for fair market value of swap	227
Series C Preferred stock redemption	(138,400)
Balance at Sep. 30, 2017	1,015,175	872	1,164,658	0	(213,771)	0	63,416
Balance at Mar. 31, 2017	1,099,746	866	1,117,628	136,144	(216,724)	(1)	61,833
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exchange of Common OP Units for Common Stock	0	1	1,085				(1,086)
Issuance of Common Stock through employee stock purchase plan	361		361
Compensation expenses related to restricted stock and stock options	2,502		2,502
Adjustment for Common OP Unitholders in the Operating Partnership	0		(152)				152
Adjustment for fair market value of swap	31					31
Consolidated net income	44,462			2,316	39,497		2,649
Distributions	(47,576)			(2,316)	(42,415)		(2,845)
Other	(115)		(116)		1
Balance at Jun. 30, 2017	1,099,411	867	1,121,308	136,144	(219,641)	30	60,703
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exchange of Common OP Units for Common Stock	0	0	5				(5)
Issuance of Common Stock through employee stock purchase plan	851		851
Issuance of stock	42,037	5	42,032					$ 0	$ 757	$ (757)
Compensation expenses related to restricted stock and stock options	2,556		2,556
Adjustment for Common OP Unitholders in the Operating Partnership	0		(2,276)				2,276
Adjustment for fair market value of swap	(30)					(30)
Consolidated net income	54,865			3,054	48,525		3,286
Distributions	(47,796)			(2,297)	(42,655)		(2,844)
Series C Preferred stock redemption								$ (136,144)		$ (136,144)
Other	(575)		(575)
Balance at Sep. 30, 2017	1,015,175	872	1,164,658	0	(213,771)	0	63,416
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle (as described in Note 2)	(15,186)				(15,186)
Adjusted balance	1,084,856	883	1,242,109	0	(227,166)	942	68,088
Balance at Dec. 31, 2017	1,100,042	883	1,242,109	0	(211,980)	942	68,088
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exchange of Common OP Units for Common Stock	0		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	0		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	883	1,245,214	0	(215,749)	2,815	67,976
Balance at Dec. 31, 2017	1,100,042	883	1,242,109	0	(211,980)	942	68,088
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock		10	78,745
Adjustment for fair market value of swap	3,017
Balance at Sep. 30, 2018	1,190,517	895	1,325,648	0	(211,743)	3,959	71,758
Balance at Mar. 31, 2018	1,101,139	883	1,245,214	0	(215,749)	2,815	67,976
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exchange of Common OP Units for Common Stock	0	1	81				(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	0		(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	884	1,248,047	0	(218,453)	3,579	67,774
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exchange of Common OP Units for Common Stock	0	1	858				(859)
Issuance of Common Stock through employee stock purchase plan	765		765
Issuance of stock	78,755
Compensation expenses related to restricted stock and stock options	2,746		(2,746)
Adjustment for Common OP Unitholders in the Operating Partnership	0		(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	$ 895	$ 1,325,648	$ 0	$ (211,743)	$ 3,959	$ 71,758