Components of Net Periodic Pension Expense Related to Qualified and Non-Qualified Plans (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
Service cost	$ 1,964	$ 1,483	$ 5,892	$ 4,609
Interest cost	1,599	1,518	4,795	4,567
Expected return on plan assets	(2,234)	(1,674)	(6,700)	(5,004)
Amortization of prior service cost	177	160	531	480
Amortization of actuarial loss	582	345	1,745	1,030
Total net periodic pension expense	2,088	1,832	6,263	5,682
Other changes in plan assets and benefit obligations recognized in other comprehensive income:	(759)	(505)	(2,276)	(1,510)
Amortization of prior service cost	(177)	(160)	(531)	(480)
Amortization of actuarial loss	(582)	(345)	(1,745)	(1,030)
Total amount recognized in net periodic benefit cost and other comprehensive income (before tax)	$ 1,329	$ 1,327	$ 3,987	$ 4,172

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

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The amounts in accumulated other comprehensive income related to gains and losses that are not recognized immediately and are expected to be recognized as components of net periodic benefit cost over the next fiscal year that follows the most recent annual statement of financial position presented.

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The amounts in accumulated other comprehensive income related to prior service cost or credit expected to be recognized as components of net periodic benefit cost over the next fiscal year that follows the most recent annual statement of financial position presented.

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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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Before tax amount, net of reclassification adjustments, of the (increase) decrease in the value of the projected benefit obligation and the increase (decrease) in the value of the plan assets resulting from experience different from that assumed or from a change in an actuarial assumption that has not been recognized in net periodic benefit cost.

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The aggregate amount in accumulated other comprehensive income expected to be recognized as a component of net periodic benefit cost over the fiscal year that follows the most recent annual statement of financial position presented, including the net gain (loss), net prior service cost or credit, and net transition asset or obligation.

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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.

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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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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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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.

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