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Retained Earnings and Regulatory Capital Requirements - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]        
Restricted cash   $ 2,475 $ 2,205  
Return of capital to parent   $ 2,400 $ 0 $ 0
HSBC Bank USA [Member]        
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]        
Return of capital to parent $ 2,400      
HNAH [Member]        
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]        
Return of capital to parent $ 2,400