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Note Employee benefits (Aggregate status of the plans and the amounts recognized in the consolidated financial statements) (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Change in fair value of plan assets:      
Fair value of plan assets at beginning of year $ 767,539    
Fair value of plan assets at end of year 685,823 $ 767,539  
Pension Plans      
Change in benefit obligation:      
Benefit obligation at beginning of year 816,988 778,658  
Service cost 0 0 $ 0
Interest cost 25,493 25,889 26,558
Termination benefit cost 0 0 0
Actuarial loss (gain) (47,549) 52,125  
Benefits paid (40,374) (39,684)  
Benefit obligation at end of year 754,558 816,988 778,658
Change in fair value of plan assets:      
Fair value of plan assets at beginning of year 767,539 697,129  
Actual return on plan assets (41,572) 93,857  
Employer contribution 230 16,237  
Benefits paid (40,374) (39,684)  
Fair value of plan assets at end of year 685,823 767,539 697,129
Amounts recognized in accumulated other comprehensive losss:      
Prior service (cost) credit 0 0  
Net loss 304,330 290,327  
Accumulated other comprehensive loss at end of year 304,330 290,327 311,166
Reconciliation of net (liabilities) assets:      
Net (liabilities) assets at beginning of year (49,449) (81,529)  
Accumulated other comprehensive loss at beginning of year 290,327 311,166  
Amount prepaid (accued) at beginning of year 240,878 229,637  
Net periodic benefit (credit) cost (5,513) (4,996) (6,761)
Additional benefit cost 0 0  
Contributions 230 16,237  
Amount prepaid (accrued) at end of year 235,595 240,878 229,637
Accumulated other comprehensive loss (304,330) (290,327) (311,166)
Net (liabilities) assets at end of year (68,735) (49,449) (81,529)
OPEB Plan      
Change in benefit obligation:      
Benefit obligation at beginning of year 170,720 162,365  
Service cost 1,028 1,026 1,156
Interest cost 5,562 5,703 6,021
Termination benefit cost 1,790 0 0
Actuarial loss (gain) (20,547) 6,983  
Benefits paid (5,138) (5,357)  
Benefit obligation at end of year 153,415 170,720 162,365
Change in fair value of plan assets:      
Fair value of plan assets at beginning of year 0 0  
Actual return on plan assets 0 0  
Employer contribution 5,138 5,357  
Benefits paid (5,138) (5,357)  
Fair value of plan assets at end of year 0 0 0
Amounts recognized in accumulated other comprehensive losss:      
Prior service (cost) credit 0 (3,470)  
Net loss 5,720 27,549  
Accumulated other comprehensive loss at end of year 5,720 24,079 13,865
Reconciliation of net (liabilities) assets:      
Net (liabilities) assets at beginning of year (170,720) (162,365)  
Accumulated other comprehensive loss at beginning of year 24,079 13,865  
Amount prepaid (accued) at beginning of year (146,641) (148,500)  
Net periodic benefit (credit) cost (4,402) (3,498) (4,476)
Additional benefit cost (1,790) 0  
Contributions 5,138 5,357  
Amount prepaid (accrued) at end of year (147,695) (146,641) (148,500)
Accumulated other comprehensive loss (5,720) (24,079) (13,865)
Net (liabilities) assets at end of year $ (153,415) $ (170,720) $ (162,365)