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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Pension Plans      
Defined Benefit Plan Disclosure [Line Items]      
Plan Assets at Beginning of Year $ 102,315 $ 91,058  
Actual Return on Plan Assets (6,149) 12,731  
Employer Contributions 16,628 4,100 $ 5,146
Benefits Paid (4,986) (5,574)  
Plan Assets at End of Year 107,808 102,315 91,058
PBO at Beginning of Year 166,921 150,439  
Service Cost 3,393 3,295 3,402
Interest Cost 5,878 6,057 5,945
Plan Amendments   608  
Benefits Paid (4,986) (5,574) (4,900)
Actuarial (Gain) or Loss (15,009) 12,096  
PBO at End of Year 156,197 166,921 150,439
Funded Status: Assets vs PBO (48,389) (64,606)  
Other Postretirement Benefit Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Plan Assets at Beginning of Year 20,234 16,606  
Actual Return on Plan Assets (1,085) 1,907  
Employer Contributions 4,000 4,000 4,000
Participant Contributions 153 126  
Benefits Paid (2,193) (2,405)  
Plan Assets at End of Year 21,109 20,234 16,606
PBO at Beginning of Year 94,122 96,659  
Service Cost 2,933 2,974 2,610
Interest Cost 3,404 3,913 3,232
Participant Contributions 153 126 61
Benefits Paid (2,193) (2,405) (2,421)
Actuarial (Gain) or Loss (17,414) (7,145)  
PBO at End of Year 81,005 94,122 96,659
Funded Status: Assets vs PBO (59,896) (73,888)  
Supplemental Employee Retirement Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Employer Contributions 401 34 34
Benefits Paid (401) (34)  
PBO at Beginning of Year 11,723 9,566  
Service Cost 487 460 162
Interest Cost 404 392 386
Benefits Paid (401) (34) (34)
Actuarial (Gain) or Loss 1,541 1,339  
PBO at End of Year 13,754 11,723 $ 9,566
Funded Status: Assets vs PBO $ (13,754) $ (11,723)