Occupancy Expense and Information Systems and Communications Expense - Summary of Lease Cost (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Finance lease:
Amortization of right-of-use assets	$ 6	$ 11
Interest on lease liabilities	3	6
Total finance lease expense	9	17
Sublease income	(3)	(5)
Net finance lease expense	6	12
Operating lease:
Operating lease expense	45	89
Sublease income	(2)	(3)
Net operating lease expense	43	86
Net lease expense	49	98
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from finance leases	3	6
Operating cash flows from operating leases	48	99
Financing cash flows from finance leases	8	39
Right-of-use assets obtained in exchange for new lease obligations:
Operating leases	4	33
Finance leases	$ 0	$ 0