Information for Financial Instruments on Which Fair Value Option was Elected (Parenthetical) (Detail)	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans and bonds contractual payment past due, number days	1 day	1 day

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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