Notes Receivable - Schedule of Aggregate LTV Ratio (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Segment Reporting [Line Items]
Aggregate LTV ratio	50.00%	50.00%	50.00%
SFS
Segment Reporting [Line Items]
Secured loans	$ 513,482	$ 590,609	$ 666,456
Low auction estimate of collateral	$ 1,295,353	$ 1,369,235	$ 1,375,569
Aggregate LTV ratio	40.00%	43.00%	48.00%
SFS | Loan to Value Ratio Above 50%
Segment Reporting [Line Items]
Secured loans	$ 121,599	$ 168,116	$ 273,895
Low auction estimate of collateral	$ 209,933	$ 269,063	$ 481,711
Aggregate LTV ratio	58.00%	62.00%	57.00%