Components of Retirement Plan Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Pension Plans
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Service Cost	$ 893	$ 807
Interest Cost	1,142	1,158
Expected Return on Plan Assets	(1,489)	(1,347)
Prior Service Cost Amortization	52	47
Actuarial Loss Amortization	1,059	904
Sub-total	1,657	1,569
Amounts Capitalized and Deferred	(616)	(557)
Net Periodic Benefit Cost Recognized	1,041	1,012
Other Postretirement Benefit Plans, Defined Benefit
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Service Cost	631	517
Interest Cost	612	576
Expected Return on Plan Assets	(180)	(174)
Prior Service Cost Amortization	425	432
Transition Obligation Amortization		5
Actuarial Loss Amortization	196	32
Sub-total	1,684	1,388
Amounts Capitalized and Deferred	(769)	(420)
Net Periodic Benefit Cost Recognized	915	968
Supplemental Employee Retirement Plans, Defined Benefit
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Service Cost	18	72
Interest Cost	60	53
Prior Service Cost Amortization	3	3
Actuarial Loss Amortization	46	16
Sub-total	127	144
Net Periodic Benefit Cost Recognized	$ 127	$ 144

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

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- Definition

The amount of the transition obligation or asset recognized in net periodic benefit 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

-Topic 715

-SubTopic 30

-Glossary Expected Return on Plan Assets

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-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

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 87

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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

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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 Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph a, h

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: 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

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 87

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-Publisher FASB

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-Number 106

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- Definition

Defined Benefit Plan, 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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