Target and Weighted Average Allocation for Pension and OPB Plans by Asset Category (Detail)	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	100.00%	100.00%
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	100.00%	100.00%
Cash Equivalents [Member] \| Pension Plan [Member]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum	0.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum	10.00%
Defined Benefit Plan, Actual Plan Asset Allocations	3.00%	3.00%
Cash Equivalents [Member] \| Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum	5.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum	15.00%
Defined Benefit Plan, Actual Plan Asset Allocations	10.00%	7.00%
Equity Securities [Member] \| Pension Plan [Member]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum	0.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum	50.00%
Defined Benefit Plan, Actual Plan Asset Allocations	47.00%	49.00%
Equity Securities [Member] \| Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum	20.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum	40.00%
Defined Benefit Plan, Actual Plan Asset Allocations	30.00%	31.00%
Debt Securities [Member] \| Pension Plan [Member]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum	50.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum	100.00%
Defined Benefit Plan, Actual Plan Asset Allocations	50.00%	48.00%
Debt Securities [Member] \| Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum	50.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum	70.00%
Defined Benefit Plan, Actual Plan Asset Allocations	60.00%	62.00%

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

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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- Definition

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.

+ 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)
-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)(1)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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