Securitized Debt Obligations, Net - Schedule of Information on Securitized Debt Obligations (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 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	$ 11.0	$ 4.7
Collateral Assets [Member] \| Single Asset Securitization [Member]
Debt Instrument [Line Items]
Total loan amount, securitized	$ 500.0

This is the principal amount outstanding for securitized loans only (across all types of loans).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719

Name:	us-gaap_PrincipalAmountOutstandingOnLoansSecuritized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant