Finance Receivables - Internal Credit Quality Rating (Details) $ in Thousands	Sep. 30, 2017 USD ($) tenant	Dec. 31, 2016 USD ($) tenant
Credit Quality Of Finanace Receivables:
Financing receivable	$ 619,011	$ 539,892
Direct Financing Method | Internally Assigned Grade 1
Credit Quality Of Finanace Receivables:
Number of tenants and obligors | tenant	0	0
Financing receivable	$ 0	$ 0
Direct Financing Method | Internally Assigned Grade 2
Credit Quality Of Finanace Receivables:
Number of tenants and obligors | tenant	2	2
Financing receivable	$ 62,551	$ 61,949
Direct Financing Method | Internally Assigned Grade 3
Credit Quality Of Finanace Receivables:
Number of tenants and obligors | tenant	8	9
Financing receivable	$ 379,168	$ 412,075
Direct Financing Method | Internally Assigned Grade 4
Credit Quality Of Finanace Receivables:
Number of tenants and obligors | tenant	8	5
Financing receivable	$ 165,515	$ 65,868
Direct Financing Method | Internally Assigned Grade 5
Credit Quality Of Finanace Receivables:
Number of tenants and obligors | tenant	1	0
Financing receivable	$ 11,777	$ 0