Consolidated Statement of Equity - USD ($) $ in Thousands	Total	Par Value	Additional Paid in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income	Total Shareholders’ Equity	Operating Partnership Noncontrolling Interest	Partially Owned Properties Noncontrolling Interest	Total Noncontrolling Interests
Balance at Dec. 31, 2017	$ 2,547,634	$ 1,814	$ 2,772,823	$ (315,417)	$ 13,952	$ 2,473,172	$ 73,844	$ 618	$ 74,462
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from sale of common shares	5,316	3	5,313			5,316
Restricted share award grants, net	933	2	872	59		933
Conversion of OP Units	0		126			126	(126)		(126)
Dividends/distributions declared	(43,126)			(41,910)		(41,910)	(1,216)		(1,216)
Preferred distributions	(487)			(487)		(487)
Distributions	(43)							(43)	(43)
Change in market value of Redeemable Noncontrolling Interest	1,470		194	1,276		1,470
Change in fair value of interest rate swap agreements	4,298				4,298	4,298
Net income	11,265			10,908		10,908	313	44	357
Adjustment for Noncontrolling Interests ownership in Operating Partnership	0		(712)			(712)	712		712
Balance at Mar. 31, 2018	2,527,260	1,819	2,778,616	(345,571)	18,250	2,453,114	73,527	619	74,146
Balance at Dec. 31, 2017	2,547,634	1,814	2,772,823	(315,417)	13,952	2,473,172	73,844	618	74,462
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Change in fair value of interest rate swap agreements	6,533
Balance at Sep. 30, 2018	2,482,631	1,822	2,785,797	(396,127)	20,485	2,411,977	69,993	661	70,654
Balance at Mar. 31, 2018	2,527,260	1,819	2,778,616	(345,571)	18,250	2,453,114	73,527	619	74,146
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from sale of common shares	654		654			654
Restricted share award grants, net	2,011		1,986	25		2,011
Purchase of OP Units	(303)						(303)		(303)
Conversion of OP Units	0	1	816			817	(817)		(817)
Dividends/distributions declared	(43,119)			(41,927)		(41,927)	(1,192)		(1,192)
Preferred distributions	(284)			(284)		(284)	0		0
Distributions	(43)							(43)	(43)
Change in market value of Redeemable Noncontrolling Interest	(39)		(39)			(39)
Change in fair value of interest rate swap agreements	1,321				1,321	1,321
Net income	11,967			11,587		11,587	331	49	380
Adjustment for Noncontrolling Interests ownership in Operating Partnership	0		(166)			(166)	166		166
Balance at Jun. 30, 2018	2,499,425	1,820	2,781,867	(376,170)	19,571	2,427,088	71,712	625	72,337
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from sale of common shares	2,084	2	2,082			2,084
Restricted share award grants, net	1,767	0	1,989	(222)		1,767
Purchase of OP Units	(1,252)						(1,252)		(1,252)
Conversion of OP Units	0	0	384			384	(384)		(384)
Dividends/distributions declared	(43,141)			(41,973)		(41,973)	(1,168)		(1,168)
Preferred distributions	(284)			(284)		(284)
Distributions	(36)							(36)	(36)
Change in market value of Redeemable Noncontrolling Interest	(570)		(96)	(474)		(570)
Change in fair value of interest rate swap agreements	914				914	914
Net income	23,724			22,996		22,996	656	72	728
Adjustment for Noncontrolling Interests ownership in Operating Partnership	0		(429)			(429)	429		429
Balance at Sep. 30, 2018	2,482,631	1,822	2,785,797	(396,127)	20,485	2,411,977	69,993	661	70,654
Balance at Dec. 31, 2018	2,447,660	1,824	2,791,555	(428,307)	14,433	2,379,505	67,477	678	68,155
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from sale of common shares	31,020	17	31,003			31,020
Restricted share award grants, net	355	2	640	(287)		355
Purchase of OP Units	(105)						(105)		(105)
Dividends/distributions declared	(43,694)			(42,536)		(42,536)	(1,158)		(1,158)
Preferred distributions	(284)			(284)		(284)
Distributions	(47)							(47)	(47)
Change in market value of Redeemable Noncontrolling Interest	(290)		(290)	0		(290)
Change in fair value of interest rate swap agreements	(3,476)				(3,476)	(3,476)
Net income	11,426			11,057		11,057	305	64	369
Adjustment for Noncontrolling Interests ownership in Operating Partnership	0		(149)			(149)	149		149
Balance at Mar. 31, 2019	2,442,326	1,843	2,822,520	(460,357)	10,957	2,374,963	66,668	695	67,363
Balance at Dec. 31, 2018	2,447,660	1,824	2,791,555	(428,307)	14,433	2,379,505	67,477	678	68,155
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Change in fair value of interest rate swap agreements	(11,296)
Balance at Sep. 30, 2019	2,463,651	1,883	2,903,459	(526,195)	3,137	2,382,284	81,052	315	81,367
Balance at Mar. 31, 2019	2,442,326	1,843	2,822,520	(460,357)	10,957	2,374,963	66,668	695	67,363
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from sale of common shares	18,316	10	18,306			18,316
Restricted share award grants, net	2,668		2,646	22		2,668
Purchase of OP Units	(717)						(717)		(717)
Dividends/distributions declared	(43,912)			(42,674)		(42,674)	(1,238)		(1,238)
Preferred distributions	(294)			(294)		(294)
Issuance of OP Units in connection with acquisition	6,488						6,488		6,488
Distributions	(47)							(47)	(47)
Change in market value of Redeemable Noncontrolling Interest	(529)		155	(684)		(529)
Buyout of Noncontrolling Interest - partially owned properties	(540)		(122)			(122)		(418)	(418)
Change in fair value of interest rate swap agreements	(5,558)				(5,558)	(5,558)
Net income	7,242			6,980		6,980	200	62	262
Adjustment for Noncontrolling Interests ownership in Operating Partnership	0		1,560			1,560	(1,560)		(1,560)
Balance at Jun. 30, 2019	2,425,443	1,853	2,845,065	(497,007)	5,399	2,355,310	69,841	292	70,133
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from sale of common shares	52,094	30	52,064			52,094
Restricted share award grants, net	2,249		2,398	(149)		2,249
Purchase of OP Units	(89)						(89)		(89)
Dividends/distributions declared	(44,645)			(43,369)		(43,369)	(1,276)		(1,276)
Preferred distributions	(314)			(314)		(314)
Issuance of OP Units in connection with acquisition	16,110						16,110		16,110
Contribution	572
Distributions	(40)							(40)	(40)
Change in market value of Redeemable Noncontrolling Interest	(441)		(36)	(405)		(441)
Buyout of Noncontrolling Interest - partially owned properties	(572)		0			0		(572)	(572)
Change in fair value of interest rate swap agreements	(2,262)				(2,262)	(2,262)
Net income	15,546			15,049		15,049	434	63	497
Adjustment for Noncontrolling Interests ownership in Operating Partnership	0		3,968			3,968	(3,968)		(3,968)
Balance at Sep. 30, 2019	$ 2,463,651	$ 1,883	$ 2,903,459	$ (526,195)	$ 3,137	$ 2,382,284	$ 81,052	$ 315	$ 81,367