Other Postretirement Benefits - Additional Information (Detail) $ in Millions	6 Months Ended
Jun. 30, 2015 USD ($)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected contributions	$ 6

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