Consolidated Statement of Equity - USD ($) $ in Thousands	Total	Cumulative effect of changes in accounting standard	Total Shareholders’  Equity	Total Shareholders’  Equity Cumulative effect of changes in accounting standard	Par Value	Additional Paid in Capital	Additional Paid in Capital Cumulative effect of changes in accounting standard	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)	Total Noncontrolling Interests	Operating Partnership Noncontrolling Interest	Partially Owned Properties Noncontrolling Interest
Balance at Dec. 31, 2018	$ 2,447,660	$ (239)	$ 2,379,505	$ (239)	$ 1,824	$ 2,791,555	$ (239)	$ (428,307)	$ 14,433	$ 68,155	$ 67,477	$ 678
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from sale of common shares	31,020		31,020		17	31,003
Restricted share award grants, net	355		355		2	640		(287)
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 in Operating Partnership	(290)		(290)			(290)
Change in fair value of interest rate swap agreements	(3,476)		(3,476)						(3,476)
Adjustment for Noncontrolling Interests ownership in Operating Partnership	0		(149)			(149)				149	149
Net income	11,426		11,057					11,057		369	305	64
Balance at Mar. 31, 2019	2,442,326		2,374,963		1,843	2,822,520		(460,357)	10,957	67,363	66,668	695
Balance at Dec. 31, 2018	2,447,660	(239)	2,379,505	(239)	1,824	2,791,555	(239)	(428,307)	14,433	68,155	67,477	678
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		2,382,284		1,883	2,903,459		(526,195)	3,137	81,367	81,052	315
Balance at Mar. 31, 2019	2,442,326		2,374,963		1,843	2,822,520		(460,357)	10,957	67,363	66,668	695
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from sale of common shares	18,316		18,316		10	18,306
Restricted share award grants, net	2,668		2,668			2,646		22
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 in Operating Partnership	(529)		(529)			155		(684)
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)
Adjustment for Noncontrolling Interests ownership in Operating Partnership	0		1,560			1,560				(1,560)	(1,560)
Net income	7,242		6,980					6,980		262	200	62
Balance at Jun. 30, 2019	2,425,443		2,355,310		1,853	2,845,065		(497,007)	5,399	70,133	69,841	292
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from sale of common shares	52,094		52,094		30	52,064
Restricted share award grants, net	2,249		2,249			2,398		(149)
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 in Operating Partnership	(441)		(441)			(36)		(405)
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)
Adjustment for Noncontrolling Interests ownership in Operating Partnership	0		3,968			3,968				(3,968)	(3,968)
Net income	15,546		15,049					15,049		497	434	63
Balance at Sep. 30, 2019	2,463,651		2,382,284		1,883	2,903,459		(526,195)	3,137	81,367	81,052	315
Balance at Dec. 31, 2019	2,480,984	(147)	2,408,948	(147)	1,900	2,931,921	(147)	(529,194)	4,321	72,036	71,697	339
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from sale of common shares	239,232		239,232		124	239,108
Restricted share award grants, net	(201)		(201)		2	245		(448)
Purchase of OP Units	(93)									(93)	(93)
Dividends/distributions declared	(47,911)		(46,636)					(46,636)		(1,275)	(1,275)
Preferred distributions	(317)		(317)					(317)
Distributions	(41)									(41)		(41)
Change in market value of Redeemable Noncontrolling Interest in Operating Partnership	(980)		(980)			581		(1,561)
Change in fair value of interest rate swap agreements	(9,986)		(9,986)						(9,986)
Adjustment for Noncontrolling Interests ownership in Operating Partnership	0		(2,038)			(2,038)				2,038	2,038
Net income	14,885		14,414					14,414		471	404	67
Balance at Mar. 31, 2020	2,675,425		2,602,289		2,026	3,169,670		(563,742)	(5,665)	73,136	72,771	365
Balance at Dec. 31, 2019	2,480,984	$ (147)	2,408,948	$ (147)	1,900	2,931,921	$ (147)	(529,194)	4,321	72,036	71,697	339
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Change in fair value of interest rate swap agreements	(10,630)
Balance at Sep. 30, 2020	2,715,183		2,643,076		2,082	3,275,187		(627,884)	(6,309)	72,107	71,676	431
Balance at Mar. 31, 2020	2,675,425		2,602,289		2,026	3,169,670		(563,742)	(5,665)	73,136	72,771	365
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from sale of common shares	99,505		99,505		55	99,450
Restricted share award grants, net	2,226		2,226			3,017		(791)
Purchase of OP Units	(67)									(67)	(67)
Dividends/distributions declared	(49,195)		(47,924)					(47,924)		(1,271)	(1,271)
Preferred distributions	(317)		(317)					(317)
Distributions	(42)									(42)		(42)
Change in market value of Redeemable Noncontrolling Interest in Operating Partnership	(579)		(579)			(416)		(163)
Change in fair value of interest rate swap agreements	(1,243)		(1,243)						(1,243)
Adjustment for Noncontrolling Interests ownership in Operating Partnership	0		(806)			(806)				806	806
Net income	18,369		17,820					17,820		549	476	73
Balance at Jun. 30, 2020	2,744,082		2,670,971		2,081	3,270,915		(595,117)	(6,908)	73,111	72,715	396
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net proceeds from sale of common shares	1,529		1,529		1	1,528
Restricted share award grants, net	2,458		2,458			2,951		(493)
Purchase of OP Units	(355)									(355)	(355)
Dividends/distributions declared	(49,214)		(47,943)					(47,943)		(1,271)	(1,271)
Preferred distributions	(317)		(317)					(317)
Distributions	(41)									(41)		(41)
Change in market value of Redeemable Noncontrolling Interest in Operating Partnership	42		42			(45)		87
Change in fair value of interest rate swap agreements	599		599						599
Adjustment for Noncontrolling Interests ownership in Operating Partnership	0		(162)			(162)				162	162
Net income	16,400		15,899					15,899		501	425	76
Balance at Sep. 30, 2020	$ 2,715,183		$ 2,643,076		$ 2,082	$ 3,275,187		$ (627,884)	$ (6,309)	$ 72,107	$ 71,676	$ 431