Segments - Summary (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total revenues	$ 179,008	$ 185,983	$ 371,946	$ 385,978
Interest expense	(14,573)	(20,119)	(29,290)	(42,746)
Depreciation and amortization	(56,966)	(55,055)	(110,317)	(111,313)
Operating expenses	(166,398)	(146,877)	(310,117)	(293,837)
Property management fees from external customers	2,288	2,253	4,902	4,663
Total consolidated revenues, including interest income	180,240	187,458	374,410	388,732
Segment operating income before depreciation, amortization and allocation of corporate overhead	12,610	39,106	61,829	92,141
(Loss) gain from disposition of real estate	(632)	0	(632)	17,409
Provision for real estate impairment	(15,317)	0	(15,317)	0
Income tax provision	(267)	(345)	(524)	(690)
Net income	(2,653)	18,765	31,796	64,974
Total assets	6,224,973		6,224,973		$ 5,865,913
Operating segments
Segment Reporting Information [Line Items]
Total revenues	184,177	191,882	382,452	398,093
Total segment assets	6,131,934	6,058,625	6,131,934	6,058,625
Segment operating income before depreciation, amortization and allocation of corporate overhead	98,111	96,846	210,137	205,094
Net income	(2,653)	18,765	31,796	64,974
Unallocated corporate assets	93,039	256,432	93,039	256,432
Total assets	6,224,973	6,315,057	6,224,973	6,315,057
Operating segments | Wholly-owned properties
Segment Reporting Information [Line Items]
Total revenues and other income	169,874	175,395	349,584	361,899
Interest income	385	268	776	533
Total revenues	170,259	175,663	350,360	362,432
Operating expenses before depreciation, amortization, ground/facility leases and allocation of corporate overhead	(72,493)	(76,705)	(144,770)	(154,703)
Ground/facility leases	(1,687)	(1,452)	(3,287)	(2,906)
Interest expense	664	(5,231)	346	(12,137)
Operating income before depreciation, amortization and allocation of corporate overhead	96,743	92,275	202,649	192,686
Depreciation and amortization	53,345	51,053	103,002	102,276
Capital expenditures	148,074	124,431	276,728	210,343
Total segment assets	6,017,795	5,942,924	6,017,795	5,942,924
Operating segments | On-campus participating properties
Segment Reporting Information [Line Items]
Total revenues and other income	6,171	6,214	16,329	16,260
Interest income	15	2	23	2
Total revenues	6,186	6,216	16,352	16,262
Operating expenses before depreciation, amortization, ground/facility leases and allocation of corporate overhead	(3,562)	(3,013)	(6,498)	(5,771)
Ground/facility leases	(778)	(1,015)	(1,535)	(1,865)
Interest expense	(1,342)	(1,419)	(2,675)	(2,837)
Operating income before depreciation, amortization and allocation of corporate overhead	504	769	5,644	5,789
Depreciation and amortization	1,869	1,831	3,729	3,654
Capital expenditures	661	409	870	1,064
Total segment assets	100,970	104,826	100,970	104,826
Operating segments | Development Services
Segment Reporting Information [Line Items]
Operating income before depreciation, amortization and allocation of corporate overhead	(2,932)	(1,264)	(6,080)	(3,824)
Total segment assets	3,187	2,088	3,187	2,088
Development and construction management fees	675	2,121	1,131	3,156
Operating expenses	(3,607)	(3,385)	(7,211)	(6,980)
Operating segments | Property Management Services
Segment Reporting Information [Line Items]
Total revenues	7,057	7,882	14,609	16,243
Operating income before depreciation, amortization and allocation of corporate overhead	3,796	5,066	7,924	10,443
Total segment assets	9,982	8,787	9,982	8,787
Operating expenses	(3,261)	(2,816)	(6,685)	(5,800)
Property management fees from external customers	2,288	2,253	4,902	4,663
Intersegment revenues	4,769	5,629	9,707	11,580
Unallocated
Segment Reporting Information [Line Items]
Total revenues	832	1,205	1,665	2,219
Operating expenses	(27,582)	(22,681)	(51,551)	(45,526)
Elimination of intersegment revenues
Segment Reporting Information [Line Items]
Total revenues	$ (4,769)	$ (5,629)	$ (9,707)	$ (11,580)