Occupancy Expense and Information Systems and Communications Expense - Summary of Lease Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Finance lease:
Amortization of right-of-use assets	$ 20	$ 21
Interest on lease liabilities	9	11
Total finance lease expense	29	32
Sublease income	(11)	(9)
Net finance lease expense	18	23
Operating lease:
Operating lease expense	169	179
Sublease income	(16)	(6)
Net operating lease expense	153	173
Net lease expense	171	196
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from finance leases	9	11
Operating cash flows from operating leases	192	201
Financing cash flows from finance leases	33	54
Right-of-use assets obtained in exchange for new lease obligations:
Operating leases	38	120
Finance leases	$ 0	$ 0