Summary of Notes Payable, Secured Revolving Repurchase Agreements, Senior Secured Credit Facility (Parenthetical) (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Repurchase Agreements, Senior Secured Credit Facility and Note Payable
Debt Instrument [Line Items]
Recourse guarantee percentage	25.00%	25.00%
Repurchase Agreements
Debt Instrument [Line Items]
Recourse guarantee percentage	100.00%	100.00%
Senior Secured Credit Facilities
Debt Instrument [Line Items]
Recourse guarantee percentage	100.00%