Finance Receivables - Internal Credit Quality Rating (Details) $ in Thousands	Mar. 31, 2018 USD ($) tenant	Dec. 31, 2017 USD ($) tenant
Credit Quality Of Finanace Receivables:
Financing receivable	$ 613,275	$ 619,728
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	$ 62,936	$ 62,744
Direct Financing Method | Internally Assigned Grade 3
Credit Quality Of Finanace Receivables:
Number of tenants and obligors | tenant	10	8
Financing receivable	$ 393,540	$ 379,621
Direct Financing Method | Internally Assigned Grade 4
Credit Quality Of Finanace Receivables:
Number of tenants and obligors | tenant	5	8
Financing receivable	$ 124,839	$ 165,413
Direct Financing Method | Internally Assigned Grade 5
Credit Quality Of Finanace Receivables:
Number of tenants and obligors | tenant	2	1
Financing receivable	$ 31,960	$ 11,950