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Goodwill and acquisitions - Summary of Movements in the Impairment Charge from a Reasonably Possible Change in Assumptions (Detail)
£ in Millions
 6 Months Ended
Jun. 30, 2020
GBP (£)
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact ​of change ​in revenue less pass-through costs growth rate ​on impairment ​charges £ 52.3
Impact ​of change ​in operating ​margin ​percentage ​on impairment ​charges 198.1
Impact of change in long-term cash flow growth rate on impairment charges 147.9
Increase [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges (591.2)
Decrease [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges 762.0
Wunderman Thompson [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact ​of change ​in revenue less pass-through costs growth rate ​on impairment ​charges 19.2
Impact ​of change ​in operating ​margin ​percentage ​on impairment ​charges 70.8
Impact of change in long-term cash flow growth rate on impairment charges 54.6
Wunderman Thompson [member] | Increase [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges (210.2)
Wunderman Thompson [member] | Decrease [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges 279.4
VMLY&R [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact ​of change ​in revenue less pass-through costs growth rate ​on impairment ​charges 8.3
Impact ​of change ​in operating ​margin ​percentage ​on impairment ​charges 37.7
Impact of change in long-term cash flow growth rate on impairment charges 22.9
VMLY&R [member] | Increase [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges (100.1)
VMLY&R [member] | Decrease [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges 132.0
Burson Cohn & Wolfe [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact ​of change ​in revenue less pass-through costs growth rate ​on impairment ​charges 9.6
Impact ​of change ​in operating ​margin ​percentage ​on impairment ​charges 21.8
Impact of change in long-term cash flow growth rate on impairment charges 26.9
Burson Cohn & Wolfe [member] | Increase [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges (98.7)
Burson Cohn & Wolfe [member] | Decrease [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges 127.0
Geometry Global [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact ​of change ​in revenue less pass-through costs growth rate ​on impairment ​charges 1.2
Impact ​of change ​in operating ​margin ​percentage ​on impairment ​charges 13.5
Impact of change in long-term cash flow growth rate on impairment charges 3.6
Geometry Global [member] | Increase [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges (22.3)
Geometry Global [member] | Decrease [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges 29.7
Landor And Fitch [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact ​of change ​in revenue less pass-through costs growth rate ​on impairment ​charges 2.1
Impact ​of change ​in operating ​margin ​percentage ​on impairment ​charges 6.5
Impact of change in long-term cash flow growth rate on impairment charges 6.0
Landor And Fitch [member] | Increase [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges (23.5)
Landor And Fitch [member] | Decrease [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges 31.3
Other [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact ​of change ​in revenue less pass-through costs growth rate ​on impairment ​charges 11.9
Impact ​of change ​in operating ​margin ​percentage ​on impairment ​charges 47.8
Impact of change in long-term cash flow growth rate on impairment charges 33.9
Other [member] | Increase [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges (136.4)
Other [member] | Decrease [member]  
Summary of Movements in the Impairment Charge from A Reasonably Possible Change In These Assumptions [Line Items]  
Impact of change to pre-tax discount rate on impairment charges £ 162.6