Pension and Other Postretirement Benefits - Schedule of Accumulated Benefit Obligation (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Qualified retirement plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 811,184	$ 699,269
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 35,362	$ 32,695

- Definition

For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.

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