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Retirement Plans - Schedule of Weighted-Average Assumptions Used to Determine Net Periodic Cost (Detail) - Net Periodic Cost [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Periodic Benefit Cost Assumptions [Line Items]
Discount rate	3.74%	3.88%	3.61%
Health care cost trend: Initial	6.00%	6.25%	6.50%
Health care cost trend: Ultimate	4.75%	4.75%	4.75%
Year ultimate reached	2022	2022	2022
Corridor	10.00%	10.00%	10.00%
Recognition period for gains and losses	11 years	11 years	11 years

X

- Definition

Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Recognized Period

+ References

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- Definition

Defined Benefit Plan Corridor Rate

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- Definition

Defined Benefit Plan Initial Health Care Cost Trend Rate

+ References

No definition available.

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- Definition

Net Periodic Benefit Cost Assumptions [Line Items]

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- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit 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 (k)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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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

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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