Postretirement Benefit Plans - Defined Contribution Plans - Additional Information (Detail)	Sep. 30, 2017 USD ($)
Defined Contribution Plans [Member]
Defined Contribution Plan Disclosure [Line Items]
Balance of trust assets	$ 1,502,000

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