Pension and Other Postretirement Benefits - Fair Values of Pension Plan Assets by Asset Category (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	$ 173,154	$ 160,806
Large-cap fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	62,151	59,693
Mid-cap fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	11,581	10,105
Small-cap fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	15,955	14,423
Government bond fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	24,385	23,270
Corporate bond fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	21,819	19,004
Fixed income fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	37,231	34,279
Cash & equivalents [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	32	32
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	32	32
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Large-cap fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mid-cap fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Small-cap fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Government bond fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate bond fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fixed income fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Cash & equivalents [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	32	32
Significant Observable Inputs (Level 2) [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	173,122	160,774
Significant Observable Inputs (Level 2) [Member] | Large-cap fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	62,151	59,693
Significant Observable Inputs (Level 2) [Member] | Mid-cap fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	11,581	10,105
Significant Observable Inputs (Level 2) [Member] | Small-cap fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	15,955	14,423
Significant Observable Inputs (Level 2) [Member] | Government bond fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	24,385	23,270
Significant Observable Inputs (Level 2) [Member] | Corporate bond fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	21,819	19,004
Significant Observable Inputs (Level 2) [Member] | Fixed income fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	37,231	34,279
Significant Unobservable Inputs (Level 3) [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Large-cap fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Mid-cap fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Small-cap fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Government bond fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Corporate bond fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Fixed income fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category
Significant Unobservable Inputs (Level 3) [Member] | Cash & equivalents [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category