Occupancy Expense and Information Systems and Communications Expense - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Occupancy Expense and Information Systems and Communications Expense [Abstract]
Depreciation	$ 526	$ 472	$ 443
Property Subject to or Available for Operating Lease [Line Items]
Capital lease obligation	159	194
Interest expense on capital lease	20	22	32
Accumulated amortization on capital lease	401	365
Sublease rental expense	229	194	190
Sublease revenue	5	$ 4	$ 4
Future minimum sublease rental revenue on capital lease	41
Future minimum sublease rental revenue on operating lease	$ 19
One Lincoln Street
Property Subject to or Available for Operating Lease [Line Items]
Leased area (in sqft) | ft²	1,025,000
Noncancelable capital leases expiration	20 years
Sublease building (in sqft) | ft²	127,000