Postretirement Benefits Other Than Pensions - Summary of Assumptions Used to Determine Benefit Obligations (Detail)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Benefit Obligation
Defined Benefit Plan Disclosure [Line Items]
Healthcare cost trend rate assumed for next year	7.70%	7.90%
Rate to which the cost trend rate is assumed to decline (ultimate trend rate)	4.50%	4.50%
Year of ultimate rate achievement	2029	2029
Weighted average discount rate	3.85%	4.64%
Net Periodic Benefit Cost
Defined Benefit Plan Disclosure [Line Items]
Healthcare cost trend rate assumed for next year	7.90%	8.20%	8.50%
Rate to which the cost trend rate is assumed to decline (ultimate trend rate)	4.50%	4.50%	4.50%
Year of ultimate rate achievement	2029	2029	2029
Weighted average discount rate	4.64%	3.38%	4.34%

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

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- Definition

The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (l)

-URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Health Care Cost Trend Rate

-URI http://asc.fasb.org/extlink&oid=6514820

+ Details

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- Definition

The ultimate trend rate for health care costs.

+ References

+ Details

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- Definition

Year the ultimate health care cost trend rate is expected to be reached, in CCYY format.

+ References

+ Details

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