Pension and Other Postretirement Benefits - Schedule of Reconciliation of Activity for Investments (Parenthetical) (Detail) (Pension and Other Postretirement Benefit Plans' Assets [Member], USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Pension and Other Postretirement Benefit Plans' Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Transfer into level 3	$ 0	$ 0
Transfer out of level 3	$ 0	$ 0

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration