Allowance for Doubtful Accounts - Notes Receivable Disaggregated by Vintage Year (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)
Current-period gross write-offs
2020		$ 0
2019		0
2018		411
2017		0
2016		1,970
Prior		40
Current-period gross write-offs	$ 253	2,421
Current-period recoveries
2020		0
2019		0
2018		0
2017		0
2016		0
Prior		0
Current-period recoveries		0
Current-period net write-offs
2020		0
2019		0
2018		411
2017		0
2016		1,970
Prior		40
Current-period net write-offs		2,421
Standard notes receivable
Financing Receivable, Credit Quality Indicator [Line Items]
2020	7,007	7,007
2019	11,716	11,716
2018	17,608	17,608
2017	26,260	26,260
2016	28	28
Prior	1,598	1,598
Total	64,217	64,217
Current-period gross write-offs
Current-period gross write-offs	253	1,974
Elevated risk notes receivable
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0	0
2019	603	603
2018	0	0
2017	5,511	5,511
2016	0	0
Prior	2,421	2,421
Total	8,535	8,535
Current-period gross write-offs
Current-period gross write-offs	$ 0	$ 447