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Pensions and Other Postretirement Benefits - Assumptions (Details)
3 Months Ended 6 Months Ended 12 Months Ended
Dec. 31, 2023
Sep. 30, 2023
Jun. 30, 2023
Mar. 31, 2023
Dec. 31, 2022
Sep. 30, 2022
Jun. 30, 2022
Mar. 31, 2022
Dec. 31, 2021
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Jun. 30, 2022
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
HSBC North America Pension Plan                                
Net Benefit Cost [Abstract]                                
Discount rate 5.65% 5.00% 4.80% 5.00% 4.75% 4.60% 3.55% 2.85% 2.80% 2.70% 3.10% 2.45%        
Expected long-term rate of return on Plan assets         4.75% 4.00%             3.50% 5.25% 4.75% 3.50%
Postretirement Plans Other Than Pensions                                
Projected Benefit Obligation [Abstract]                                
Discount rate 4.80%       4.95%       2.70%         4.80% 4.95% 2.70%
Salary increase assumption 3.00%       3.00%       3.00%         3.00% 3.00% 3.00%