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Pensions and Other Postretirement Benefits-Assumptions (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
HSBC North America Pension Plan [Member]      
Net Benefit Cost [Abstract]      
Discount rate 3.60% 4.25% 3.95%
Expected long-term rate of return on Plan assets 5.50% 5.50% 6.00%
Postretirement Plans Other Than Pensions [Member]      
Projected Benefit Obligation [Abstract]      
Discount rate 3.45% 3.95% 3.95%
Salary increase assumption 3.00% 3.00% 3.00%
Net Benefit Cost [Abstract]      
Discount rate 3.95% 3.95% 3.60%
Salary increase assumption 3.00% 3.00% 3.00%