Occupancy Expense and Information Systems and Communications Expense - Summary of Lease Cost (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Finance lease:
Amortization of right-of-use assets	$ 5	$ 16
Interest on lease liabilities	3	9
Total finance lease expense	8	25
Sublease income	(2)	(7)
Net finance lease expense	6	18
Operating lease:
Operating lease expense	43	132
Sublease income	(1)	(4)
Net operating lease expense	42	128
Net lease expense	48	146
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from finance leases	2	8
Operating cash flows from operating leases	43	142
Financing cash flows from finance leases	8	47
Right-of-use assets obtained in exchange for new lease obligations:
Operating leases	21	54
Finance leases	$ 0	$ 0