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Employee compensation and benefits - Key actuarial assumptions for the principal plan (Details) - Pension Plan - Principal plan
Dec. 31, 2019
Dec. 31, 2018
Disclosure of defined benefit plans [line items]    
Discount rate 2.00% 2.80%
Inflation rate 3.10% 3.40%
Rate of increase for pensions 2.90% 3.10%
Rate of pay increase 3.65% 3.65%