Allowance for Doubtful Accounts - Notes Receivable Disaggregated by Vintage Year (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)
Current-period gross write-offs
2020		$ 0
2019		0
2018		411
2017		0
2016		1,721
Prior		36
Current-period gross write-offs	$ 2,168	2,168
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		1,721
Prior		36
Current-period net write-offs		2,168
Standard notes receivable
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8,127	8,127
2019	13,619	13,619
2018	17,669	17,669
2017	26,429	26,429
2016	275	275
Prior	1,602	1,602
Total	67,721	67,721
Current-period gross write-offs
Current-period gross write-offs	1,721	1,721
Elevated risk notes receivable
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0	0
2019	604	604
2018	0	0
2017	5,511	5,511
2016	0	0
Prior	2,421	2,421
Total	8,536	8,536
Current-period gross write-offs
Current-period gross write-offs	$ 447	$ 447