Occupancy Expense and Information Systems and Communications Expense - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
Lease liabilities	$ 1,176	$ 1,176
Depreciation and amortization	206	411
Finance lease right-of-use asset	88	88
Finance lease liability	152	152
Interest on lease liabilities	3	6
Accumulated amortization of finance lease right-of-use asset	46	46
Right-of-use assets	876	876
Lease liabilities	1,024	1,024
Operating leases not yet commenced	$ 497	$ 497
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	11 years	11 years
Maximum
Lessee, Lease, Description [Line Items]
Operating leases not yet commenced, lease terms	15 years	15 years