Notes Receivable and Allowance for Losses - Schedule of notes receivable (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total notes receivable	$ 124,884	$ 95,529
Allowance for losses on receivables specifically evaluated for impairment	4,426	1,647
Allowance for losses on non-impaired loans	259	490
Total loan reserves	4,685	2,137
Net carrying value	120,199	93,392
Current portion, net	36,759	13,256
Long-term portion, net	83,440	80,136
Senior
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total notes receivable	94,349	73,700
Subordinated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total notes receivable	28,100	18,647
Unsecured
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total notes receivable	$ 2,435	$ 3,182