Subsequent Events - Additional Information (Detail)	12 Months Ended
Dec. 31, 2017
Wells Fargo [Member] \| Customer Agreement [Member]
Subsequent Event [Line Items]
Customer agreement termination date	Mar. 26, 2018

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

No definition available.

Name:	us-gaap_SubsequentEventLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration