Investment Securities - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Transfers from Level 2 to Level 1	$ 2,416	$ 11,032

Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

Name:	us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant