Notes Receivable and Allowance for Losses - Past due balances of mezzanine and other notes receivable (Details) - Mezzanine & Other Notes Receivable - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	$ 0	$ 0
Current	124,884	95,529
Total Notes Receivable	124,884	95,529
Senior
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Current	94,349	73,700
Total Notes Receivable	94,349	73,700
Subordinated
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Current	28,100	18,647
Total Notes Receivable	28,100	18,647
Unsecured
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Current	2,435	3,182
Total Notes Receivable	2,435	3,182
Financing Receivables, 30 to 89 Days Past Due
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, 30 to 89 Days Past Due | Senior
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, 30 to 89 Days Past Due | Subordinated
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, 30 to 89 Days Past Due | Unsecured
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Senior
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Subordinated
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Unsecured
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	$ 0	$ 0