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Goodwill and acquisitions - Summary of Movements in the Impairment Charge from a Reasonably Possible Change in Assumptions (Parenthetical) (Detail)
 6 Months Ended
Jun. 30, 2020
Disclosure Of Reasonably Possible Change in Assumptions Used in the Impairment Review [Abstract]  
Reasonably possible increase in projected growth rate of revenue less pass-through costs 0.50%
Reasonably possible decrease in projected growth rate of revenue less pass-through costs 0.50%
Reasonably possible percentage increase in projected operating margin 0.50%
Reasonably possible percentage decrease in projected operating margin 0.50%
Reasonably possible percentage increase in projected long term cash flow growth rate 0.50%
Reasonably possible percentage decrease in projected long term cash flow growth rate 0.50%
Reasonably possible percentage increase in pre-tax discount rate 1.50%
Reasonably possible percentage decrease in pre-tax discount rate 1.50%