Strategic Initiatives - Branch Closings (Details) $ in Millions	3 Months Ended				6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($) branch	Sep. 30, 2020 USD ($)	Mar. 31, 2020 branch	Jun. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 branch	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Restructuring Cost and Reserve [Line Items]
Number of branches closing | branch		30		60				20
Notional value of derivative contracts	$ 1,606,786					$ 1,606,786				$ 2,021,133
Interest rate contracts [Member]
Restructuring Cost and Reserve [Line Items]
Notional value of derivative contracts	375,427					375,427				$ 509,403
Write Down of Lease Right-of-use Assets, Net of Estimated Sublease Income [Member] | Occupancy Expense, Net [Member]
Restructuring Cost and Reserve [Line Items]
Lease impairment charges		$ 29			$ 46
Write Down of Leasehold Improvement Assets [Member] | Occupancy Expense, Net [Member]
Restructuring Cost and Reserve [Line Items]
Fixed asset impairment charges		18			$ 16
Write Down of Equipment Assets [Member] | Occupancy Expense, Net [Member]
Restructuring Cost and Reserve [Line Items]
Fixed asset impairment charges		$ 3
Write-down of Lease Right-of-use Assets and Leasehold Improvement Assets Associated with Exit of Office Space [Member] | Occupancy Expense, Net [Member]
Restructuring Cost and Reserve [Line Items]
Lease impairment charges	4					9
Lease Termination and Associated Costs [Member] | Occupancy Expense, Net [Member]
Restructuring Cost and Reserve [Line Items]
Lease impairment charges			$ 7
Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Trading losses associated with exit of certain derivative contracts	22		47			32	$ 57
Allocated restructuring costs from affiliates	13		$ 13			43	$ 55
Restructuring Plan [Member] | Interest rate contracts [Member]
Restructuring Cost and Reserve [Line Items]
Notional value of derivative contracts	20,400					20,400
Restructuring Plan [Member] | Forecast [Member] | Interest rate contracts [Member]
Restructuring Cost and Reserve [Line Items]
Notional value of derivative contracts									$ 33,900
HSBC Group Restructuring Activities [Member]
Restructuring Cost and Reserve [Line Items]
Allocated restructuring costs from affiliates	$ 14					$ 32