Occupancy Expense and Information Systems and Communications Expense - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
Lease liabilities	$ 994
Depreciation and amortization	858	$ 842	$ 599
Repositioning charge	51
Impairment, right of use assets	46
One-time repairs	5
Finance lease right-of-use asset	55	78
Finance lease liability	103	136
Finance lease, accumulated amortization	75
Interest on lease liabilities	9	11
Right-of-use assets	$ 720	$ 858
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:AccruedLiabilities
Lease liabilities	$ 891
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssets
Operating leases not yet commenced	$ 462
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Right-of-use assets				$ 910
Lease liabilities				$ 1,060
Minimum
Lessee, Lease, Description [Line Items]
Operating leases not yet commenced, lease terms	10 years
Maximum
Lessee, Lease, Description [Line Items]
Operating leases not yet commenced, lease terms	15 years