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Assumed Health Care Cost Trend Rates (Detail) - Pre Sixty Five Coverage	12 Months Ended
	Dec. 31, 2017
Schedule of Pension Expected Future Benefit Payments [Line Items]
Accumulated postretirement benefit obligation	7.60%
Net periodic postretirement benefit obligation	7.60%
Ultimate health care cost trend	4.54%
Ultimate year health care cost trend rate is achieved	2038
Net periodic postretirement benefit obligation December 31, 2018	7.45%

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- Definition

Defined Benefit Plan Assumed Health Care Cost Trend Net Periodic Postretirement Benefit Cost

+ References

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- Definition

Defined Benefit Plan Assumed Health Care Cost Trend Postretirement Benefit Obligation

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- Definition

Defined Benefit Plan, Ultimate Health Care Cost Trend Rate, Postretirement Benefit Obligation

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- Definition

Defined Benefit Plan Year That Rate Reaches Ultimate Trend Rate Postretirement Benefit Obligation

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- Definition

Schedule Of Pension Expected Future Benefit Payments [Line Items]

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- Definition

Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (l)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

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