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Summary of Information on Plans' Assets, Projected Benefit Obligations (PBO), and Funded Status (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension Plans      
Defined Benefit Plan Disclosure [Line Items]      
Plan Assets at Beginning of Year $ 107,808 $ 102,315  
Actual Return on Plan Assets 17,908 (6,149)  
Employer Contributions 6,916 16,628 $ 4,100
Participant Contributions 0    
Benefits Paid (6,877) (4,986)  
Plan Assets at End of Year 125,755 107,808 102,315
PBO at Beginning of Year 156,197 166,921  
Service Cost 3,104 3,393 3,295
Interest Cost 6,484 5,878 6,057
Plan Amendments 0    
Benefits Paid (6,877) (4,986) (5,574)
Actuarial (Gain) or Loss 23,227 (15,009)  
PBO at End of Year 182,135 156,197 166,921
Funded Status: Assets vs PBO (56,380) (48,389)  
Other Postretirement Benefit Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Plan Assets at Beginning of Year 21,109 20,234  
Actual Return on Plan Assets 3,808 (1,085)  
Employer Contributions 4,000 4,000 4,000
Participant Contributions 121 153  
Benefits Paid (1,758) (2,193)  
Plan Assets at End of Year 27,280 21,109 20,234
PBO at Beginning of Year 81,005 94,122  
Service Cost 2,304 2,933 2,974
Interest Cost 3,426 3,404 3,913
Plan Amendments 0    
Benefits Paid (1,758) (2,193) (2,405)
Actuarial (Gain) or Loss 10,559 (17,414)  
PBO at End of Year 95,657 81,005 94,122
Funded Status: Assets vs PBO (68,377) (59,896)  
Supplemental Employee Retirement Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Plan Assets at Beginning of Year 0    
Actual Return on Plan Assets 0    
Employer Contributions 610 401 34
Participant Contributions 0    
Benefits Paid (610) (401)  
Plan Assets at End of Year 0 0  
PBO at Beginning of Year 13,754 11,723  
Service Cost 247 487 460
Interest Cost 567 404 392
Plan Amendments 225    
Benefits Paid (610) (401) (34)
Actuarial (Gain) or Loss 3,576 1,541  
PBO at End of Year 17,759 13,754 $ 11,723
Funded Status: Assets vs PBO $ (17,759) $ (13,754)