Retirement Plans - Actuarial Assumptions Used by Corporation's Postretirement Medical Plan (Detail) (Postretirement Medical Plan [Member])	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Postretirement Medical Plan [Member]
Assumptions used to determine benefit obligations at December 31
Discount rate	3.10%	3.60%	3.10%
Initial health care trend rate	6.80%	7.10%	7.30%
Ultimate trend rate	4.50%	4.60%	4.80%
Year in which ultimate trend rate is reached	2029	2027	2022

Year in which ultimate trend rate is reached.

No definition available.

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- Definition

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

+ References

+ Details

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- Details

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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=35743120&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

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- Definition

The ultimate trend rate for health care costs.

+ References

+ Details

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