Summary of Secured Revolving Repurchase Agreements, Senior Secured and Secured Credit Agreements, and Asset-Specific Financings (Parenthetical) (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Asset-specific Financing, Secured Revolving Repurchase Agreements and Senior Secured and Secured Credit Agreements
Debt Instrument [Line Items]
Recourse guarantee percentage	25.00%	25.00%
Repurchase Agreements
Debt Instrument [Line Items]
Recourse guarantee percentage	100.00%	100.00%
Secured Credit Agreement
Debt Instrument [Line Items]
Recourse guarantee percentage	100.00%	100.00%