Securitized Debt Obligations, Net - Schedule of Information on Securitized Debt Obligations (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Percentage of loans financed by securitized debt obligations earned floating rate of interest			98.00%		98.00%
Interest expense on securitized debt obligations	$ 12.5	$ 3.8	$ 35.6	$ 3.8
Collateral Assets [Member] | 2017 Single Asset Securitization [Member]
Debt Instrument [Line Items]
Total loan amount, securitized	$ 500.0		$ 500.0