Finance Receivables - Internal Credit Quality Rating (Details) $ in Thousands	Sep. 30, 2018 USD ($) tenant	Dec. 31, 2017 USD ($) tenant
Credit Quality Of Finance Receivables
Carrying Value	$ 105,653	$ 106,457
Internally Assigned Grade 1
Credit Quality Of Finance Receivables
Number of tenants and obligors | tenant	0	0
Carrying Value	$ 0	$ 0
Internally Assigned Grade 2
Credit Quality Of Finance Receivables
Number of tenants and obligors | tenant	2	2
Carrying Value	$ 15,725	$ 14,386
Internally Assigned Grade 3
Credit Quality Of Finance Receivables
Number of tenants and obligors | tenant	2	2
Carrying Value	$ 29,742	$ 29,716
Internally Assigned Grade 4
Credit Quality Of Finance Receivables
Number of tenants and obligors | tenant	2	2
Carrying Value	$ 60,186	$ 62,355
Internally Assigned Grade 5
Credit Quality Of Finance Receivables
Number of tenants and obligors | tenant	0	0
Carrying Value	$ 0	$ 0