Segments - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 255,835	$ 227,705	$ 217,371	$ 242,131	$ 245,873	$ 213,469	$ 201,059	$ 220,409	$ 943,042	$ 880,810	$ 796,447
Ground/facility lease expense									(14,151)	(11,855)	(10,213)
Interest expense, net									(111,287)	(99,228)	(71,122)
Operating expenses									(763,152)	(668,215)	(654,541)
Depreciation, Depletion and Amortization, Nonproduction									(275,046)	(263,203)	(234,955)
Total consolidated revenues, including interest income									946,728	885,644	801,392
Segment income before depreciation and amortization	55,743	27,307	37,841	58,999	67,520	21,501	73,168	50,406	179,890	212,595	141,906
Loss (gain) from disposition of real estate									(53)	42,314	(632)
Other operating and nonoperating income									0	2,648	0
Provision for impairment									(17,214)	0	(15,317)
Gain from extinguishment of debt									20,992	7,867	0
Income tax provision									(1,507)	(2,429)	(989)
Net income	25,848	$ 19,336	$ 10,210	$ 31,368	49,621	$ (2,737)	$ 45,990	$ 26,250	86,762	119,124	70,121
Total assets	7,559,754				7,038,846				7,559,754	7,038,846	6,897,370
Operating segments
Segment Reporting Information [Line Items]
Total revenues									943,682	882,379	798,057
Depreciation, Depletion and Amortization, Nonproduction									(270,318)	(258,534)	(231,476)
Segment income before depreciation and amortization									482,131	447,102	416,883
Total assets	7,466,613				6,951,652				7,466,613	6,951,652	6,806,091
Operating segments | Owned Properties
Segment Reporting Information [Line Items]
Rental revenues and other income									880,709	829,119	741,909
Interest income									473	1,436	1,545
Total revenues									881,182	830,555	743,454
Operating expenses before depreciation, amortization, and ground/facility lease expense									(390,664)	(373,521)	(332,429)
Ground/facility lease expense									(11,084)	(8,927)	(7,372)
Interest expense, net									(16,859)	(14,742)	(3,659)
Income before depreciation and amortization									462,575	433,365	399,994
Depreciation, Depletion and Amortization, Nonproduction									(261,938)	(250,715)	223,940
Capital expenditures									514,043	546,147	617,552
Total assets	7,346,625				6,841,222				7,346,625	6,841,222	6,691,758
Operating segments | On-Campus Participating Properties
Segment Reporting Information [Line Items]
Rental revenues and other income									36,346	34,596	33,945
Interest income									167	133	65
Total revenues									36,513	34,729	34,010
Operating expenses before depreciation, amortization, and ground/facility lease expense									(15,028)	(14,602)	(14,384)
Ground/facility lease expense									(3,067)	(2,928)	(2,841)
Interest expense, net									(4,934)	(5,098)	(5,264)
Income before depreciation and amortization									13,484	12,101	11,521
Depreciation, Depletion and Amortization, Nonproduction									(8,380)	(7,819)	7,536
Capital expenditures									2,898	3,654	3,533
Total assets	97,561				93,917				97,561	93,917	100,031
Operating segments | Development Services
Segment Reporting Information [Line Items]
Development and construction management fees									13,051	7,281	10,761
Operating expenses									(8,658)	(8,031)	(7,618)
Income before depreciation and amortization									4,393	(750)	3,143
Total assets	13,539				10,087				13,539	10,087	6,726
Operating segments | Property Management Services
Segment Reporting Information [Line Items]
Operating expenses									(11,257)	(7,428)	(7,607)
Income before depreciation and amortization									1,679	2,386	2,225
Total assets	8,888				6,426				8,888	6,426	7,576
Unallocated
Segment Reporting Information [Line Items]
Total revenues									3,046	3,265	3,335
Operating expenses									(117,529)	(110,660)	(90,250)
Depreciation and amortization									(4,728)	(4,669)	(3,479)
Total assets	$ 93,141				$ 87,194				93,141	87,194	91,279
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Other operating and nonoperating income									0	3,949	0
Amortization of deferred financing costs									(5,012)	(5,816)	(4,619)
Third-party management services | Operating segments | Property Management Services
Segment Reporting Information [Line Items]
Total revenues									$ 12,936	$ 9,814	$ 9,832