Rent and Tenant Receivables and Other Assets, Net (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Straight-line rent receivable, net	$ 284,003,000		$ 284,003,000		$ 278,831,000
Accounts receivable, net	47,229,000		47,229,000		53,051,000
Deferred costs, net	3,500,000		3,500,000		5,185,000
Investment in direct financing leases, net	5,806,000		5,806,000		6,547,000
Investment in Retained REITs	7,929,000		7,929,000		7,255,000
Prepaid expenses	5,646,000		5,646,000		3,850,000
Leasehold improvements, property and equipment, net	3,398,000		3,398,000		3,991,000
Other assets, net	7,675,000		7,675,000		7,910,000
Total	365,186,000		365,186,000		366,620,000
Unrealized gain (loss) included in consolidated statement of operations			700,000	$ (700,000)
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Amortization expense	100,000	$ 100,000	300,000	300,000
Write off of leasehold	0	0	0	0
Accumulated amortization	3,600,000		3,600,000		3,400,000
Property and Equipment [Member]
Property, Plant and Equipment [Line Items]
Accumulated amortization	7,000,000.0		7,000,000.0		6,500,000
Depreciation expense	300,000	300,000	500,000	600,000
Line of Credit [Member]
Property, Plant and Equipment [Line Items]
Amortization expense	800,000	$ 800,000	1,700,000	$ 1,500,000
Accumulated amortization for deferred costs	$ 54,600,000		$ 54,600,000		$ 52,900,000