Employee Benefits - Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost (Parenthetical) (Detail) - Net Periodic Benefit Cost Assumptions [Member]	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Prior to Age 65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed rate decrease	5.00%	5.00%	5.00%
Health care cost trend rate to decrease Period	2025	2019	2019
After Age 65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed rate decrease	5.00%	5.00%	5.00%
Health care cost trend rate to decrease Period	2025	2019	2019
Pension Plans [Member] \| Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed plan duration	15 years 6 months	15 years
Pension Plans [Member] \| Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed plan duration	12 years 1 month 6 days	11 years 10 months 25 days
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed plan duration	6 years 2 months 12 days	6 years 3 months 19 days

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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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 net periodic benefit cost discount rate support bond indices, duration.

+ References

No definition available.

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