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Retirement plans - Assumptions Used to Determine Defined Benefit Plans Costs (Detail) - Retirement Plans
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Discount rate (as a percent) 4.34% 3.64% 4.12%
Expected return on plan assets (as a percent) 6.75% 7.00% 7.00%