Consolidated Statements of Changes In Equity - USD ($) $ in Thousands	Total	Common Stock	Paid-in Capital	6.75% Series C Cumulative Redeemable Perpetual Preferred Stock	Distributions in Excess of Accumulated Earnings	Non- controlling interests – Common OP Units	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2014	$ 979,027	$ 838	$ 1,029,601	$ 136,144	$ (254,209)	$ 67,034	$ (381)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of Common OP Units to Common Stock	0		225			(225)
Issuance of Common Stock through exercise of options	3,816	2	3,814
Issuance of Common Stock through employee stock purchase plan	1,083		1,083
Compensation expenses related to restricted stock	8,582		8,582
Repurchase of Common Stock or Common OP Units	(3,201)		(3,201)
Adjustment for Common OP Unitholders in the Operating Partnership	0		(496)			496
Adjustment for fair market value of swap	(172)						(172)
Net income	150,512			9,226	130,145	11,141
Distributions	(146,465)			(9,226)	(126,416)	(10,823)
Other	(491)	3	468		(26)
Balance at Dec. 31, 2015	992,691	843	1,039,140	136,144	(250,506)	67,623	(553)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of Common OP Units to Common Stock	0		381			(381)
Issuance of Common Stock through exercise of options	11,288	4	11,284
Issuance of Common Stock through employee stock purchase plan	1,269		1,269
Issuance of Common Stock	50,000	7	49,993
Compensation expenses related to restricted stock	9,181		9,181
Repurchase of Common Stock or Common OP Units	(2,652)		(2,652)
Adjustment for Common OP Unitholders in the Operating Partnership	0		(4,426)			4,426
Adjustment for fair market value of swap	326						326
Net income	187,132			9,226	164,037	13,869
Distributions	(166,266)			(9,226)	(144,807)	(12,233)
Other	(1,122)		(1,122)
Balance at Dec. 31, 2016	1,081,847	854	1,103,048	136,144	(231,276)	73,304	(227)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of Common OP Units to Common Stock	0	13	16,436			(16,449)
Issuance of Common Stock through exercise of options	4,850	2	4,848
Issuance of Common Stock through employee stock purchase plan	2,061		2,061
Issuance of Common Stock	120,698	14	120,684
Compensation expenses related to restricted stock	9,352		9,352
Repurchase of Common Stock or Common OP Units	(3,087)		(3,087)
Adjustment for Common OP Unitholders in the Operating Partnership	0		(10,043)			10,043
Adjustment for fair market value of swap	1,169						1,169
Net income	210,377			7,685	189,904	12,788
Distributions	(188,964)			(6,928)	(170,608)	(11,428)
Series C Preferred stock redemption	(136,144)			(136,144)
Series C Preferred stock original issuance costs	0		757	(757)
Other	(2,117)		(1,947)			(170)
Balance at Dec. 31, 2017	$ 1,100,042	$ 883	$ 1,242,109	$ 0	$ (211,980)	$ 68,088	$ 942