Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 824	$ 851	$ 2,471	$ 2,553
Interest Cost	1,514	1,486	4,543	4,458
Expected Return on Plan Assets	(1,826)	(1,814)	(5,479)	(5,442)
Prior Service Cost Amortization	66	66	197	198
Actuarial Loss Amortization	1,165	1,099	3,496	3,297
Sub-total	1,743	1,688	5,228	5,064
Amounts Capitalized and Deferred	(932)	(839)	(2,402)	(2,246)
Net Periodic Benefit Cost Recognized	811	849	2,826	2,818
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	744	652	2,231	1,956
Interest Cost	978	808	2,935	2,424
Expected Return on Plan Assets	(337)	(301)	(1,010)	(903)
Prior Service Cost Amortization	350	372	1,049	1,116
Actuarial Loss Amortization	524	262	1,573	786
Sub-total	2,259	1,793	6,778	5,379
Amounts Capitalized and Deferred	(1,226)	(904)	(3,418)	(2,508)
Net Periodic Benefit Cost Recognized	1,033	889	3,360	2,871
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	115	41	345	123
Interest Cost	98	96	294	288
Prior Service Cost Amortization	47	47	141	141
Actuarial Loss Amortization	74	94	222	282
Sub-total	334	278	1,002	834
Net Periodic Benefit Cost Recognized	$ 334	$ 278	$ 1,002	$ 834

Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan.

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- Definition

Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan.

+ References

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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.

+ References

No definition available.

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- Definition

Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan.

+ References

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- Definition

Amount of cost recognized for passage of time related to 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 (h)(2)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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

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- Definition

Amount of net periodic benefit cost (credit) for defined benefit plan.

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- Definition

Amount of cost for actuarial present value of benefits attributed to service rendered by employee for 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 (h)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amounts of pension and other post retirement benefit costs capitalized and deferred.

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No definition available.

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- Definition

The gross amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments).

+ References

No definition available.

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