Other Postretirement Benefit Plans - Assumptions Used in Valuation of Other Postretirement Benefits (Detail)	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Compensation And Retirement Disclosure [Abstract]
Weighted average discount rate	3.95%	3.00%	3.95%	3.00%

The interest rate used to adjust for the time value of money.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(1)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

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