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Summary of Information on Plans' Assets, Projected Benefit Obligations (PBO), and Funded Status (Detail) (USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Pension Plans
      
Schedule of Defined Benefit Plan Change in Benefit Obligation [Line Items]      
Service Cost $ 3,227,000 $ 2,941,000 $ 2,608,000
Interest Cost 4,633,000 4,684,000 4,457,000
Curtailment Gain     (41,000)
Funded Status: Assets vs PBO (44,081,000) (43,019,000)  
Pension Plans | Change in Plan Assets
      
Schedule of Defined Benefit Plan Change in Benefit Obligation [Line Items]      
Plan Assets at Beginning of Year 59,700,000 54,100,000  
Actual Return on Plan Assets 7,780,000 225,000  
Employer Contributions 9,387,000 8,813,000  
Acquired Plan Assets        
Benefits Paid (4,456,000) (3,438,000)  
Plan Assets at End of Year 72,411,000 59,700,000  
Pension Plans | Change In Projected Benefit Obligation
      
Schedule of Defined Benefit Plan Change in Benefit Obligation [Line Items]      
PBO at Beginning of Year 102,719,000 89,393,000  
Service Cost 3,227,000 2,941,000  
Interest Cost 4,633,000 4,684,000  
Plan Amendments 617,000    
Curtailment Gain        
Benefits Paid (4,456,000) (3,438,000)  
Actuarial (Gain) or Loss 9,752,000 9,139,000  
PBO at End of Year 116,492,000 102,719,000  
Other Postretirement Benefit Plans, Defined Benefit
      
Schedule of Defined Benefit Plan Change in Benefit Obligation [Line Items]      
Service Cost 2,066,000 1,918,000 1,466,000
Interest Cost 2,303,000 2,279,000 2,016,000
Employer Contributions 2,190,000   3,482,000
Participant Contributions 18,000 13,000  
Benefits Paid (2,083,000) (1,644,000) (1,848,000)
Funded Status: Assets vs PBO (53,791,000) (43,591,000)  
Other Postretirement Benefit Plans, Defined Benefit | Change in Plan Assets
      
Schedule of Defined Benefit Plan Change in Benefit Obligation [Line Items]      
Plan Assets at Beginning of Year 7,339,000 8,862,000  
Actual Return on Plan Assets 837,000 108,000  
Employer Contributions 2,190,000    
Participant Contributions 18,000 13,000  
Acquired Plan Assets        
Benefits Paid (2,083,000) (1,644,000)  
Plan Assets at End of Year 8,301,000 7,339,000  
Other Postretirement Benefit Plans, Defined Benefit | Change In Projected Benefit Obligation
      
Schedule of Defined Benefit Plan Change in Benefit Obligation [Line Items]      
PBO at Beginning of Year 50,930,000 43,344,000  
Service Cost 2,066,000 1,918,000  
Interest Cost 2,303,000 2,279,000  
Participant Contributions 18,000 13,000  
Plan Amendments (318,000)    
Curtailment Gain        
Benefits Paid (2,083,000) (1,644,000)  
Actuarial (Gain) or Loss 9,176,000 5,020,000  
PBO at End of Year 62,092,000 50,930,000  
Supplemental Employee Retirement Plans, Defined Benefit
      
Schedule of Defined Benefit Plan Change in Benefit Obligation [Line Items]      
Service Cost 289,000 285,000 285,000
Interest Cost 211,000 227,000 227,000
Employer Contributions 53,000 53,000 53,000
Benefits Paid (53,000) (53,000) (53,000)
Funded Status: Assets vs PBO (6,207,000) (4,615,000)  
Supplemental Employee Retirement Plans, Defined Benefit | Change in Plan Assets
      
Schedule of Defined Benefit Plan Change in Benefit Obligation [Line Items]      
Employer Contributions 53,000 53,000  
Acquired Plan Assets        
Benefits Paid (53,000) (53,000)  
Supplemental Employee Retirement Plans, Defined Benefit | Change In Projected Benefit Obligation
      
Schedule of Defined Benefit Plan Change in Benefit Obligation [Line Items]      
PBO at Beginning of Year 4,615,000 4,263,000  
Service Cost 289,000 285,000  
Interest Cost 211,000 227,000  
Curtailment Gain        
Benefits Paid (53,000) (53,000)  
Actuarial (Gain) or Loss 1,145,000 (107,000)  
PBO at End of Year $ 6,207,000 $ 4,615,000