Investments In Direct Financing Leases (Narrative) (Details)		12 Months Ended
	May 22, 2015 properties	Dec. 31, 2017 USD ($) properties years	Dec. 31, 2016 USD ($) properties	Dec. 31, 2015 USD ($)	May 21, 2015 USD ($)
Initial direct costs		$ 800,000	$ 1,300,000
Allowance for lease losses			0
Investment in direct financing leases, net		57,903,000	102,698,000		$ 4,700,000
Proceeds from Sale of Lease Receivables		0	20,951,000	$ 4,741,000
Mortgage Notes and Related Accrued Interest Receivable, Net		$ 970,749,000	613,978,000
number of properties sold | properties	1	9
Gain (Loss) on Contract Termination			0
Net Investment in Direct Financing and Sales Type Leases, Carrying Value of Investments Sold		$ 91,300,000
Gain (Loss) on Disposition of Property Plant Equipment		0	0
Impairment charges		$ 10,195,000	$ 0	$ 0
Minimum [Member]
Length of lease (in years) | years		14
Maximum [Member]
Length of lease (in years) | years		16
Imagine Schools [Member]
Number of public charter school properties | properties		6	12
Allowance for lease losses		$ 7,300,000
Mortgage Notes and Related Accrued Interest Receivable, Net		$ 70,300,000
number of properties sold | properties		7
Number of properties subject to lease amendment		6
Impairment charges		$ 9,600,000
Tangible Asset Impairment Charges		$ 2,300,000
Third Parties [Member]
number of properties sold | properties		2
Imagine Madison Avenue [Member] | Imagine Schools [Member]
Tangible Asset Impairment Charges		$ 600,000