Components of Retirement Plan Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Plans, Defined Benefit
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Service Cost	$ 807	$ 735	$ 2,420	$ 2,206
Interest Cost	1,158	1,171	3,475	3,513
Expected Return on Plan Assets	(1,347)	(1,210)	(4,042)	(3,630)
Prior Service Cost Amortization	47	62	141	187
Actuarial Loss Amortization	904	783	2,712	2,349
Sub-total	1,569	1,541	4,706	4,625
Amounts Capitalized and Deferred	(703)	(874)	(1,984)	(2,055)
Net Periodic Benefit Cost Recognized	866	667	2,722	2,570
Other Postretirement Benefit Plans, Defined Benefit
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Service Cost	517	479	1,550	1,438
Interest Cost	576	570	1,727	1,709
Expected Return on Plan Assets	(174)	(204)	(521)	(613)
Prior Service Cost Amortization	432	432	1,296	1,296
Transition Obligation Amortization	5	5	16	16
Actuarial Loss Amortization	32		97
Sub-total	1,388	1,282	4,165	3,846
Amounts Capitalized and Deferred	(571)	(683)	(1,551)	(1,317)
Net Periodic Benefit Cost Recognized	817	599	2,614	2,529
Supplemental Employee Retirement Plans, Defined Benefit
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Service Cost	72	71	217	214
Interest Cost	53	57	158	170
Prior Service Cost Amortization	3	3	9	8
Actuarial Loss Amortization	16	19	47	59
Sub-total	144	150	431	451
Net Periodic Benefit Cost Recognized	$ 144	$ 150	$ 431	$ 451

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.

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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

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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.

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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.

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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

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

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- Definition

Defined Benefit Plan, Capitalized and Deferred

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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).

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No definition available.

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