Finance Receivables - Internal Credit Quality Rating (Details) $ in Thousands	Sep. 30, 2018 USD ($) tenant	Dec. 31, 2017 USD ($) tenant
Credit Quality Of Finance Receivables
Net investments in direct financing leases	$ 711,802	$ 731,578
Internally Assigned Grade1-3
Credit Quality Of Finance Receivables
Number of tenants | tenant	26	24
Net investments in direct financing leases	$ 628,744	$ 608,101
Internally Assigned Grade 4
Credit Quality Of Finance Receivables
Number of tenants | tenant	6	8
Net investments in direct financing leases	$ 83,058	$ 123,477
Internally Assigned Grade 5
Credit Quality Of Finance Receivables
Number of tenants | tenant	0	0
Net investments in direct financing leases	$ 0	$ 0