Occupancy Expense and Information Systems and Communications Expense - Summary of Lease Cost (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Finance lease:
Amortization of right-of-use assets	$ 5
Interest on lease liabilities	3
Total finance lease expense	8
Sublease income	(2)
Net finance lease expense	6
Operating lease:
Operating lease expense	44
Sublease income	(1)
Net operating lease expense	43
Net lease expense	49
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from finance leases	3
Operating cash flows from operating leases	51
Financing cash flows from finance leases	31
Right-of-use assets obtained in exchange for new lease obligations:
Operating leases	29
Finance leases	$ 0