Pension and other postretirement benefits - weighted average assumptions used in determining net periodic benefit cost (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.20%	4.50%	4.10%
Rate of return	8.10%	8.10%	8.10%
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.10%	4.30%	4.00%
Initial healthcare trend rate	7.90%	8.40%	7.30%
Ultimate healthcare trend rate	5.00%	5.00%	5.00%