Occupancy Expense and Information Systems and Communications Expense - Summary of Future Minimum Lease Payments Under Non-Cancelable Capital and Operating Leases (Details) $ in Millions	Dec. 31, 2017 USD ($)
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital leases due 2018	$ 53
Capital leases due 2019	45
Capital leases due 2020	45
Capital leases due 2021	45
Capital leases due 2022	45
Capital leases due thereafter	34
Capital leases due	267
Less amount representing interest payments	(56)
Present value of minimum lease payments	211
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Operating leases due 2018	197
Operating leases due 2019	175
Operating leases due 2020	154
Operating leases due 2021	144
Operating leases due 2022	125
Operating leases due thereafter	336
Operating leases due	1,131
Total operating and capital leases due 2018	250
Total operating and capital leases due 2019	220
Total operating and capital leases due 2020	199
Total operating and capital leases due 2021	189
Total operating and capital leases due 2022	170
Total operating and capital leases due thereafter	370
Total operating and capital leases due	$ 1,398