Notes Receivable and Allowance for Losses - Origination year and credit quality indicators (Details) $ in Thousands	Dec. 31, 2020 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 0
2019	31,158
2018	27,141
Prior	81,018
Total	139,317
Senior
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	28,643
2018	15,200
Prior	60,873
Total	104,716
Subordinated
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	2,515
2018	11,360
Prior	19,359
Total	33,234
Unsecured
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	581
Prior	786
Total	$ 1,367