Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Plans
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Service Cost	$ 916	$ 751	$ 1,832	$ 1,502
Interest Cost	1,343	1,273	2,686	2,546
Expected Return on Plan Assets	(1,694)	(1,561)	(3,388)	(3,122)
Prior Service Cost Amortization	54	53	108	106
Actuarial Loss Amortization	1,180	712	2,360	1,424
Sub-total	1,799	1,228	3,598	2,456
Amounts Capitalized and Deferred	(875)	(501)	(1,583)	(844)
Net Periodic Benefit Cost Recognized	924	727	2,015	1,612
Other Postretirement Benefit Plans, Defined Benefit
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Service Cost	656	497	1,312	994
Interest Cost	729	672	1,458	1,344
Expected Return on Plan Assets	(273)	(230)	(546)	(460)
Prior Service Cost Amortization	420	420	840	840
Actuarial Loss Amortization	288	14	576	28
Sub-total	1,820	1,373	3,640	2,746
Amounts Capitalized and Deferred	(881)	(593)	(1,642)	(1,065)
Net Periodic Benefit Cost Recognized	939	780	1,998	1,681
Supplemental Employee Retirement Plans, Defined Benefit
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Service Cost	28	14	56	28
Interest Cost	78	68	156	136
Prior Service Cost Amortization	3	3	6	6
Actuarial Loss Amortization	84	25	168	50
Sub-total	193	110	386	220
Net Periodic Benefit Cost Recognized	$ 193	$ 110	$ 386	$ 220

The amount of gains or losses recognized in net periodic benefit cost.

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- Definition

The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation.

+ 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=58740216&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Prior Service Cost

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- Definition

An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Expected Return on Plan Assets

-URI http://asc.fasb.org/extlink&oid=6512171

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

+ 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=58740216&loc=d3e1928-114920

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- Definition

The total 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 (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

+ 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)

-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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- Definition

The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Service Cost (Component of Net Periodic Pension Cost)

-URI http://asc.fasb.org/extlink&oid=6525008

Reference 2: 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=58740216&loc=d3e1928-114920

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

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-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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- Definition

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

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

+ References

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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