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Note Employee benefits (Components of net periodic benefit cost) (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Pension Plans      
Personnel costs:      
Service cost $ 0 $ 0 $ 0
Other operating expenses      
Interest cost 25,493 25,889 26,558
Expected return on plan assets (40,240) (42,752) (40,646)
Amortization of prior service cost (credit) 0 0 0
Recognized net acturial loss 20,260 21,859 20,849
Net periodic benefit (credit) cost 5,513 4,996 6,761
Termination benefit cost 0 0 0
Total net periodic pension cost 5,513 4,996 6,761
OPEB Plan      
Personnel costs:      
Service cost 1,028 1,026 1,156
Other operating expenses      
Interest cost 5,562 5,703 6,021
Expected return on plan assets 0 0 0
Amortization of prior service cost (credit) (3,470) (3,800) (3,800)
Recognized net acturial loss 1,282 569 1,099
Net periodic benefit (credit) cost 4,402 3,498 4,476
Termination benefit cost 1,790 0 0
Total net periodic pension cost $ 6,192 $ 3,498 $ 4,476