Summary of Significant Accounting Policies - Narrative (Details)		1 Months Ended	12 Months Ended
	Jan. 01, 2019 USD ($) lease	Jun. 30, 2017 USD ($)	Dec. 31, 2019 USD ($) Entity Property	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant Accounting Policies [Line Items]
Number of properties subject to ground leases | lease	28
Right of use assets			$ 460,857,000
Lease liability			473,070,000
Capitalized interest			12,100,000	$ 11,700,000	$ 15,900,000
Provision for impairment			17,214,000	0	15,317,000
Unamortized discount on loans receivable			2,300,000	2,400,000
Impairment of loans receivable			0
Deferred finance costs			$ 3,500,000	3,500,000
Number of third-party joint venture partners | Entity			6
Number of properties | Property			167
Amount of pre-development costs deferred			$ 7,100,000
Advertising costs			$ 15,700,000	13,600,000	12,700,000
In-place leases
Significant Accounting Policies [Line Items]
Lease term			1 year
Property tax abatement
Significant Accounting Policies [Line Items]
Unamortized tax abatement			$ 38,600,000	56,300,000
Amortization expense of acquired intangible assets			$ 3,500,000	3,700,000	3,300,000
Weighted average tax abatement period			18 years
Other assets
Significant Accounting Policies [Line Items]
Provision for impairment			$ 14,000,000.0
Mortgages
Significant Accounting Policies [Line Items]
Provision for impairment		$ 15,300,000
Amortization of debt discounts (premiums)			(4,900,000)	(5,300,000)	(7,800,000)
Net unamortized debt premiums			6,400,000	11,600,000
Carrying Amount
Significant Accounting Policies [Line Items]
Loans receivable			50,553,000	54,611,000
Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
Right of use assets	$ 278,200,000
Lease liability	$ 277,500,000
Owned Properties
Significant Accounting Policies [Line Items]
Provision for Loan, Lease, and Other Losses			7,300,000	7,100,000	6,400,000
Owned Properties | Mortgages
Significant Accounting Policies [Line Items]
Net unamortized debt premiums			6,596,000	11,579,000
On-campus participating properties, net
Significant Accounting Policies [Line Items]
Provision for Loan, Lease, and Other Losses			$ 500,000	$ 300,000	$ 300,000
Number of properties | Property			6
Owned Properties Held For Sale
Significant Accounting Policies [Line Items]
Provision for impairment			$ 3,200,000
Wholly Owned Properties [Member]
Significant Accounting Policies [Line Items]
Provision for impairment		$ 15,300,000
Six Joint Ventures
Significant Accounting Policies [Line Items]
Number of properties | Property			11
Corporate Office Headquarters
Significant Accounting Policies [Line Items]
Lessee, Operating, Number Of Asset Under Lease | lease	2