Segment Reporting - Additional Information (Details)	12 Months Ended
Dec. 31, 2017 customer Segment	Dec. 31, 2016 customer	Dec. 31, 2015 customer
Revenue, Major Customer [Line Items]
Number of reportable segments \| Segment	1
Consolidated Net Revenue \| Customer Concentration Risk
Revenue, Major Customer [Line Items]
Number of borrowers or investors who accounted for more than 10% \| customer	0	0	0

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

No definition available.

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration