Employee Benefit Plans - Additional Information (Detail) $ in Millions	Mar. 31, 2018 USD ($)
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Expected contribution	$ 18.9
OPEB
Defined Benefit Plan Disclosure [Line Items]
Expected contribution	$ 1.0

Amount of contributions received and expected to be received by defined benefit plan from employer in current fiscal year.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920

Name:	us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsCurrentFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant