Consolidated Statements of Changes in Capital - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019
Increase (Decrease) in Partners' Capital [Roll Forward]
Cumulative effect of changes in accounting standard									$ (239)
Contributions	$ 572
Distributions	(40)	$ (47)	$ (47)	$ (36)	$ (43)	$ (43)
Buyout of Noncontrolling Interest - partially owned properties	(572)	(540)
Change in fair value of interest rate swap agreements	(2,262)	(5,558)	(3,476)	914	1,321	4,298	$ (11,296)	$ 6,533
Net income	15,546	7,242	11,426	23,724	11,967	11,265
Operating Partnership
Increase (Decrease) in Partners' Capital [Roll Forward]
Beginning balance	2,425,443	2,442,326	2,447,660	2,499,425	2,527,260	2,547,634	2,447,660	2,547,634
Cumulative effect of changes in accounting standard									(239)
Net proceeds from sale of Trust common shares and issuance of common units	52,094	18,316	31,020	2,084	654	5,316
Trust restricted share award grants, net	2,249	2,668	355	1,767	2,011	933
Purchase of OP Units	(89)	(717)	(105)	(1,252)	(303)
Conversion of OP Units				0	0	0
OP Units - distributions	(44,645)	(43,912)	(43,694)	(43,141)	(43,119)	(43,126)
Preferred distributions	(314)	(294)	(284)	(284)	(284)	(487)
Issuance of OP Units in connection with acquisitions	16,110	6,488
Contributions	572
Distributions	(40)	(47)	(47)	(36)	(43)	(43)
Change in market value of Redeemable Limited Partners	(441)	(529)	(290)	(96)	(39)	194
Buyout of Noncontrolling Interest - partially owned properties	(572)	(540)		(474)		1,276
Change in fair value of interest rate swap agreements	(2,262)	(5,558)	(3,476)	914	1,321	4,298	(11,296)	6,533
Net income	15,546	7,242	11,426	23,724	11,967	11,265
Adjustments for Limited Partners ownership in Operating Partnership	0	0	0	0	0	0
Ending balance	2,463,651	2,425,443	2,442,326	2,482,631	2,499,425	2,527,260	2,463,651	2,482,631
Operating Partnership | Accumulated Other Comprehensive Income
Increase (Decrease) in Partners' Capital [Roll Forward]
Beginning balance	5,399	10,957	14,433	19,571	18,250	13,952	14,433	13,952
Buyout of Noncontrolling Interest - partially owned properties	0	0
Change in fair value of interest rate swap agreements	(2,262)	(5,558)	(3,476)	914	1,321	4,298
Ending balance	3,137	5,399	10,957	20,485	19,571	18,250	3,137	20,485
Operating Partnership | General Partner
Increase (Decrease) in Partners' Capital [Roll Forward]
Beginning balance	2,350,033	2,364,006	2,365,072	2,407,517	2,434,864	2,459,220	2,365,072	2,459,220
Cumulative effect of changes in accounting standard									(239)
Net proceeds from sale of Trust common shares and issuance of common units	52,094	18,316	31,020	2,084	654	5,316
Trust restricted share award grants, net	2,249	2,668	355	1,767	2,011	933
Conversion of OP Units				384	817	126
OP Units - distributions	(43,369)	(42,674)	(42,536)	(41,973)	(41,927)	(41,910)
Preferred distributions	(314)	(294)	(284)	(284)	(284)	(487)
Change in market value of Redeemable Limited Partners	(441)	(529)	(290)	(96)	(39)	194
Buyout of Noncontrolling Interest - partially owned properties	0	0		(474)		1,276
Net income	15,049	6,980	11,057	22,996	11,587	10,908
Adjustments for Limited Partners ownership in Operating Partnership	3,846	1,560	(149)	(429)	(166)	(712)
Ending balance	2,379,147	2,350,033	2,364,006	2,391,492	2,407,517	2,434,864	2,379,147	2,391,492
Operating Partnership | Limited Partner
Increase (Decrease) in Partners' Capital [Roll Forward]
Beginning balance	69,719	66,668	67,477	71,712	73,527	73,844	67,477	73,844
Purchase of OP Units	(89)	(717)	(105)	(1,252)	(303)
Conversion of OP Units				(384)	(817)	(126)
OP Units - distributions	(1,276)	(1,238)	(1,158)	(1,168)	(1,192)	(1,216)
Issuance of OP Units in connection with acquisitions	16,110	6,488
Buyout of Noncontrolling Interest - partially owned properties	0	(122)
Net income	434	200	305	656	331	313
Adjustments for Limited Partners ownership in Operating Partnership	(3,846)	(1,560)	149	429	166	712
Ending balance	81,052	69,719	66,668	69,993	71,712	73,527	81,052	69,993
Operating Partnership | Total Partners’ Capital
Increase (Decrease) in Partners' Capital [Roll Forward]
Beginning balance	2,425,151	2,441,631	2,446,982	2,498,800	2,526,641	2,547,016	2,446,982	2,547,016
Cumulative effect of changes in accounting standard									$ (239)
Net proceeds from sale of Trust common shares and issuance of common units	52,094	18,316	31,020	2,084	654	5,316
Trust restricted share award grants, net	2,249	2,668	355	1,767	2,011	933
Purchase of OP Units	(89)	(717)	(105)	(1,252)	(303)
OP Units - distributions	(44,645)	(43,912)	(43,694)	(43,141)	(43,119)	(43,126)
Preferred distributions	(314)	(294)	(284)	(284)	(284)	(487)
Issuance of OP Units in connection with acquisitions	16,110	6,488
Change in market value of Redeemable Limited Partners	(441)	(529)	(290)	(96)	(39)	194
Buyout of Noncontrolling Interest - partially owned properties	0	(122)		(474)		1,276
Change in fair value of interest rate swap agreements	(2,262)	(5,558)	(3,476)	914	1,321	4,298
Net income	15,483	7,180	11,362	23,652	11,918	11,221
Ending balance	2,463,336	2,425,151	2,441,631	2,481,970	2,498,800	2,526,641	2,463,336	2,481,970
Partially Owned Properties Noncontrolling Interest
Increase (Decrease) in Partners' Capital [Roll Forward]
Distributions	(40)	(47)	(47)	(36)	(43)	(43)
Buyout of Noncontrolling Interest - partially owned properties	(572)	(418)
Net income	63	62	64	72	49	44
Partially Owned Properties Noncontrolling Interest | Operating Partnership
Increase (Decrease) in Partners' Capital [Roll Forward]
Beginning balance	292	695	678	625	619	618	678	618
Contributions	572
Distributions	(40)	(47)	(47)	(36)	(43)	(43)
Buyout of Noncontrolling Interest - partially owned properties	(572)	(418)
Net income	63	62	64	72	49	44
Ending balance	$ 315	$ 292	$ 695	$ 661	$ 625	$ 619	$ 315	$ 661