Summary of Unrealized Gains (Losses) Related to Equity Securities (Detail) $ in Thousands	3 Months Ended
Mar. 31, 2018 USD ($)
Investments Debt And Equity Securities [Abstract]
Net gains (losses) recognized during the period on equity securities	$ (371)
Unrealized gains (losses) recognized during the reporting period on equity securities still held at the reporting date	$ (371)

Amount of unrealized gain (loss) recognized in the income statement on unsold other securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Name:	us-gaap_UnrealizedGainLossOnSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration