Retirement Plans and Other Postretirement Benefits - Summary of Corporation's Pension Plan Asset Allocation (Detail)	Dec. 31, 2017	Dec. 31, 2016
Summary of corporation's pension plan asset allocation
Assets Category, Total	100.00%	100.00%
Equity Securities [Member]
Summary of corporation's pension plan asset allocation
Assets Category, Total	64.00%	61.00%
Debt Securities [Member]
Summary of corporation's pension plan asset allocation
Assets Category, Total	35.00%	38.00%
Other [Member]
Summary of corporation's pension plan asset allocation
Assets Category, Total	1.00%	1.00%

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant