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