Schedule of Real Estate and Accumulated Depreciation - Changes in investments in real estate and related accumulated depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Depreciation:
Balance, end of year	$ (1,537,100)
On-campus participating properties, net
Investments in Real Estate:
Balance, beginning of year	162,562	$ 159,996	$ 162,929
Acquisition of land for development	0	0	0
Acquisition of properties	0	0	0
Improvements and development expenditures	2,900	3,654	3,544
Write-off of fully depreciated or damaged assets	(306)	(1,088)	(6,477)
Provision for real estate impairment	0	0	0
Disposition of real estate	0	0	0
Transfer of property from owned to OCPP structure	4,343	0	0
Balance, end of year	169,499	162,562	159,996
Accumulated Depreciation:
Balance, beginning of year	(84,925)	(78,192)	(77,132)
Depreciation for the year	(8,380)	(7,819)	(7,536)
Write-off of fully depreciated or damaged assets	306	1,086	6,476
Disposition of properties	0	0	0
Transfer of property from owned to OCPP structure	(1,312)	0	0
Balance, end of year	(94,311)	(84,925)	(78,192)
Owned Properties
Investments in Real Estate:
Balance, beginning of year	7,813,959	7,485,391	6,316,470
Acquisition of land for development	10,219	26,758	24,049
Acquisition of properties	0	0	618,183
Improvements and development expenditures	484,949	549,635	621,793
Write-off of fully depreciated or damaged assets	(3,831)	(16,758)	(40,923)
Provision for real estate impairment	(3,201)	0	(15,317)
Disposition of real estate	(160,248)	(231,067)	(38,864)
Transfer of property from owned to OCPP structure	(4,343)	0	0
Balance, end of year	8,137,504	7,813,959	7,485,391
Accumulated Depreciation:
Balance, beginning of year	(1,230,562)	(1,035,027)	(864,106)
Depreciation for the year	(255,796)	(242,123)	(213,660)
Write-off of fully depreciated or damaged assets	3,831	16,242	37,761
Disposition of properties	38,426	30,346	4,978
Transfer of property from owned to OCPP structure	1,312	0	0
Balance, end of year	$ (1,442,789)	$ (1,230,562)	$ (1,035,027)