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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, 2015
Dec. 31, 2014
Dec. 31, 2013
Pension Plans      
Schedule of Defined Benefit Plan Change in Benefit Obligation [Line Items]      
Plan Assets at Beginning of Year $ 86,744 $ 82,551  
Actual Return on Plan Assets 645 4,248  
Employer Contributions 4,215 4,191 $ 3,700
Benefits Paid (4,410) (4,246) (3,764)
Plan Assets at End of Year 87,194 86,744 82,551
PBO at Beginning of Year 136,662 108,295  
Service Cost 3,689 3,006 3,573
Interest Cost 5,392 5,092 4,567
Plan Amendments 474    
Benefits Paid (4,410) (4,246) (3,764)
Actuarial (Gain) or Loss (991) 24,515  
PBO at End of Year 140,816 136,662 108,295
Funded Status: Assets vs PBO (53,622) (49,918)  
Other Postretirement Benefit Plans, Defined Benefit      
Schedule of Defined Benefit Plan Change in Benefit Obligation [Line Items]      
Plan Assets at Beginning of Year 12,840 10,829  
Actual Return on Plan Assets (214) 486  
Employer Contributions 4,000 3,650 3,280
Participant Contributions 63 59 36
Benefits Paid (2,515) (2,184) (1,942)
Plan Assets at End of Year 14,174 12,840 10,829
PBO at Beginning of Year 73,923 56,899  
Service Cost 2,622 1,988 2,523
Interest Cost 2,918 2,686 2,448
Participant Contributions 63 59 36
Benefits Paid (2,515) (2,184) (1,942)
Actuarial (Gain) or Loss (762) 14,475  
PBO at End of Year 76,249 73,923 56,899
Funded Status: Assets vs PBO (62,075) (61,083)  
Supplemental Employee Retirement Plans, Defined Benefit      
Schedule of Defined Benefit Plan Change in Benefit Obligation [Line Items]      
Employer Contributions 40 53 53
Benefits Paid (40) (53) (53)
PBO at Beginning of Year 7,965 5,857  
Service Cost 120 57 73
Interest Cost 330 272 241
Plan Amendments 608    
Benefits Paid (40) (53) (53)
Actuarial (Gain) or Loss 194 1,832  
PBO at End of Year 9,177 7,965 $ 5,857
Funded Status: Assets vs PBO $ (9,177) $ (7,965)