Asset Allocations of Postretirement Benefit Plans (Detail)	12 Months Ended
Dec. 31, 2016
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets, range minimum	0.00%
Defined benefit plan, target allocation percentage of assets, range maximum	20.00%
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets, range minimum	15.00%
Defined benefit plan, target allocation percentage of assets, range maximum	65.00%
Fixed income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets, range minimum	10.00%
Defined benefit plan, target allocation percentage of assets, range maximum	60.00%
Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets, range minimum	0.00%
Defined benefit plan, target allocation percentage of assets, range maximum	15.00%
Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets, range minimum	0.00%
Defined benefit plan, target allocation percentage of assets, range maximum	10.00%
Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets, range minimum	0.00%
Defined benefit plan, target allocation percentage of assets, range maximum	20.00%
Commodities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets, range minimum	0.00%
Defined benefit plan, target allocation percentage of assets, range maximum	25.00%

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.

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- Definition

Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis.

+ References

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)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&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)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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- Definition

Target allocation minimum percentage of investments of this type to total plan assets presented on a weighted average basis.

+ 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 (d)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920

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