Strategic Initiatives - Branch Closings (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($) branch	Dec. 31, 2020 USD ($)	Dec. 31, 2019 branch	Dec. 31, 2021 USD ($)
Restructuring Cost and Reserve [Line Items]
Number of branches closing | branch		60		20
Notional value of derivative contracts	$ 1,751,830		$ 2,021,133
Interest rate contracts [Member]
Restructuring Cost and Reserve [Line Items]
Notional value of derivative contracts	366,868		509,403
Occupancy Expense, Net [Member]
Restructuring Cost and Reserve [Line Items]
Fixed asset impairment charges		$ 16
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		52	$ 67
2-year Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Trading losses associated with exit of certain derivative contracts	4
Allocated restructuring costs from affiliates	12	$ 8
2-year Restructuring Plan [Member] | Interest rate contracts [Member]
Restructuring Cost and Reserve [Line Items]
Notional value of derivative contracts	14,800
2-year Restructuring Plan [Member] | Forecast [Member] | Interest rate contracts [Member]
Restructuring Cost and Reserve [Line Items]
Notional value of derivative contracts					$ 59,800
HSBC Group Restructuring Activities [Member]
Restructuring Cost and Reserve [Line Items]
Allocated restructuring costs from affiliates	$ 7