Related Party Transactions - Summary of Loans (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
HSBC Finance Corporation [Member]
Related Party Transaction [Line Items]
Loans and leases receivable from related parties	$ 0	$ 2,501
HMUS [Member]
Related Party Transaction [Line Items]
Loans and leases receivable from related parties	6,690	563
HSBC Mexico [Member]
Related Party Transaction [Line Items]
Loans and leases receivable from related parties	0	195
Other short-term affiliate lending [Member]
Related Party Transaction [Line Items]
Loans and leases receivable from related parties	60	15
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Loans and leases receivable from related parties	$ 6,750	$ 3,274