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	Preferred Stock Paid-in Capital	Preferred Stock Redeemable Perpetual Preferred Stock
Balance at Dec. 31, 2015	$ 992,691	$ 843	$ 1,039,140	$ 136,144	$ (250,506)	$ (553)	$ 67,623
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exchange of Common OP Units for 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 stock	50,000	7	49,993
Compensation expenses related to stock options and 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
Consolidated 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)	(227)	73,304
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exchange of Common OP Units for common stock	0	13	16,436			0	(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 stock	120,698	14	120,684					$ 757	$ (757)
Compensation expenses related to stock options and 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
Consolidated 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)
Other	(2,117)		(1,947)				(170)
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	1	1,024				(1,025)
Issuance of common stock through exercise of options	3,823	2	3,821
Issuance of common stock through employee stock purchase plan	2,043		2,043
Issuance of stock	78,755	10	78,745
Compensation expenses related to stock options and restricted stock	9,995		9,995
Repurchase of common stock or Common OP Units	(3,011)		(3,011)
Adjustment for Common OP Unitholders in the Operating Partnership	0		(3,684)				3,684
Adjustment for fair market value of swap	1,357					1,357
Consolidated net income	226,386			16	212,596		13,774
Distributions	(209,209)			(16)	(196,464)		(12,729)
Other	(1,651)		(1,651)
Balance at Dec. 31, 2018	$ 1,193,344	$ 896	$ 1,329,391	$ 0	$ (211,034)	$ 2,299	$ 71,792