Notes Receivable and Allowance for Credit Losses - Credit Quality Indicator (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Notes Receivable and Allowance for Losses [Line Items]
2020	$ 0
2019	30,736
2018	26,950
Prior	82,026
Total	139,712	$ 133,014
Senior
Notes Receivable and Allowance for Losses [Line Items]
2020	0
2019	28,297
2018	15,432
Prior	61,863
Total	105,592	98,545
Subordinated
Notes Receivable and Allowance for Losses [Line Items]
2020	0
2019	2,439
2018	10,981
Prior	19,247
Total	32,667	32,153
Unsecured
Notes Receivable and Allowance for Losses [Line Items]
2020	0
2019	0
2018	537
Prior	916
Total	$ 1,453	$ 2,316