Notes Receivable and Allowance for Losses - Past due balances of mezzanine and other notes receivable (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Mezzanine & Other Notes Receivable
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	$ 0	$ 0
Current	95,015	74,436
Total Mezzanine and Other Notes Receivables	95,015	74,436
Mezzanine & Other Notes Receivable | Senior
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Current	73,186	61,482
Total Mezzanine and Other Notes Receivables	73,186	61,482
Mezzanine & Other Notes Receivable | Subordinated
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Current	18,647	9,336
Total Mezzanine and Other Notes Receivables	18,647	9,336
Mezzanine & Other Notes Receivable | Unsecured
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Current	3,182	3,618
Total Mezzanine and Other Notes Receivables	3,182	3,618
Forgivable Notes Receivable
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	1,615	1,465
Current	74,470	50,010
Total Mezzanine and Other Notes Receivables	76,085	51,475
Financing Receivables, 30 to 89 Days Past Due | Mezzanine & Other Notes Receivable
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, 30 to 89 Days Past Due | Mezzanine & Other Notes Receivable | Senior
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, 30 to 89 Days Past Due | Mezzanine & Other Notes Receivable | Subordinated
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, 30 to 89 Days Past Due | Mezzanine & Other Notes Receivable | Unsecured
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, 30 to 89 Days Past Due | Forgivable Notes Receivable
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	6	116
Financing Receivables, Equal to Greater than 90 Days Past Due | Mezzanine & Other Notes Receivable
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Mezzanine & Other Notes Receivable | Senior
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Mezzanine & Other Notes Receivable | Subordinated
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Mezzanine & Other Notes Receivable | Unsecured
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Forgivable Notes Receivable
Financing Receivable, Recorded Investment [Line Items]
Total Past Due	$ 1,609	$ 1,349