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Pension and other postretirement benefits - weighted average assumptions used in determining net periodic benefit cost (Detail)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Pension      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.50% 4.10% 5.00%
Rate of return 8.10% 8.10% 8.30%
Other Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.30% 4.00% 4.70%
Initial healthcare trend rate 8.40% 7.30% 7.50%
Ultimate healthcare trend rate 5.00% 5.00% 5.00%