Loans - Components of Loans (Details) - USD ($) $ in Millions	Sep. 30, 2018		Dec. 31, 2017		Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 66,756		$ 72,563		$ 67,593
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	47,183		53,035
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	11,882		10,533		10,442
Commercial Loans [Member] | Business and Corporate Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	12,731		12,504		13,328
Commercial Loans [Member] | Global Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	18,672	[1],[2]	20,088	[1],[2]	19,950
Commercial Loans [Member] | Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	3,898	[1]	9,910	[1]	4,408
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	19,573		19,528
Consumer Loans [Member] | Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	17,376		17,273		17,196
Consumer Loans [Member] | Home Equity Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,027		1,191		1,234
Consumer Loans [Member] | Credit Cards [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	907		721		656
Consumer Loans [Member] | Other consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 263		$ 343		$ 379

[1] Includes loans to HSBC affiliates which totaled $1,557 million and $6,750 million at September 30, 2018 and December 31, 2017, respectively. See Note 14, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.

[2] Represents large multinational firms including globally focused U.S. corporate and financial institutions, U.S. dollar lending to multinational banking clients managed by HSBC on a global basis and complex large business clients supported by Global Banking and Markets relationship managers.