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Assumed Health Care Cost Trend Rates (Detail) - Pre Sixty Five Coverage	12 Months Ended
	Mar. 31, 2017
Schedule of Pension Expected Future Benefit Payments [Line Items]
Accumulated postretirement benefit obligation	7.60%
Net periodic postretirement benefit obligation	7.75%
Ultimate health care cost trend	4.54%
Ultimate year health care cost trend rate is achieved	2038
Net periodic postretirement benefit obligation March 31, 2017	7.60%

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- Definition

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

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No definition available.

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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

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
-Glossary Health Care Cost Trend Rate
-URI http://asc.fasb.org/extlink&oid=6514820

Reference 2: 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=65897934&loc=d3e1928-114920

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