Employee Benefits - Weighted Average Assumptions to Determine Projected Benefit Obligations (Parenthetical) (Detail) - Projected Benefit Obligation Assumptions [Member]	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Assumed rate decrease	5.00%	5.00%
Health care cost trend rate to decrease Period	2025	2025
Pension Plans [Member] \| Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed plan duration	15 years 9 months 18 days	15 years 6 months
Pension Plans [Member] \| Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed plan duration	12 years 3 months 19 days	12 years 1 month 6 days
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed plan duration	6 years 1 month 6 days	6 years 2 months 12 days

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- Definition

Ultimate trend rate for health care cost for defined benefit postretirement plan.

+ 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=82843861&loc=d3e1928-114920

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- Definition

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

+ References

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- Definition

Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation Discount Rate Support Bond Indices, Duration

+ References

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