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Note Pension and postretirement benefits (Components of net periodic pension cost) (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]        
Amortization of net losses $ (5,386) $ (5,606) $ (16,157) $ (16,819)
Pension Plan        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost 6,029 6,120 18,086 18,359
Expected return on plan assets (9,551) (10,186) (28,653) (30,557)
Amortization of net losses 4,716 5,053 14,147 15,160
Total net periodic pension cost (benefit) 1,194 987 3,580 2,962
Benefit Restoration Plans        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost 344 352 1,033 1,057
Expected return on plan assets (509) (502) (1,527) (1,508)
Amortization of net losses 349 411 1,048 1,233
Total net periodic pension cost (benefit) $ 184 $ 261 $ 554 $ 782