Employee Benefits - Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost (Detail) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Health care cost trend rate
One percent increase	$ 0.00	$ 0.00	$ 0.00
One percent decrease	$ 0.00	$ 0.00	$ 0.00
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.97%	4.07%	5.07%
Expected return on plan assets	7.50%	7.50%	8.00%
Rate of compensation increase	4.02%	4.08%	4.05%
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.93%	3.10%	4.30%
Expected return on plan assets	1.50%	1.50%	2.25%
Prior to Age 65 [Member] \| Net Periodic Benefit Cost Assumptions [Member]
Health care cost trend rate
Health care cost trend rate	7.50%	8.00%	8.00%
After Age 65 [Member] \| Net Periodic Benefit Cost Assumptions [Member]
Health care cost trend rate
Health care cost trend rate	7.50%	8.00%	12.00%

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

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- Definition

An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Glossary Expected Long-Term Rate of Return on Plan Assets

-URI http://asc.fasb.org/extlink&oid=6512013

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(3)

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

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- Definition

Expected rate of compensation increases (for pay-related plans).

+ References

+ Details

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- Details

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- Details

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- Definition

Amount of effect of a one-percentage-point decrease in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs.

+ References

+ Details

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- Definition

The effect of a one-percentage-point increase in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs.

+ References

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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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