Occupancy Expense and Information Systems and Communications Expense - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
Lease liabilities	$ 1,121	$ 1,121
Depreciation and amortization	214	625
Finance lease right-of-use asset	84	84
Finance lease liability	144	144
Interest on lease liabilities	3	9
Accumulated amortization of finance lease right-of-use asset	51	51
Right-of-use assets	834	834
Lease liabilities	977	977
Operating leases not yet commenced	$ 508	$ 508
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