Occupancy Expense and Information Systems and Communications Expense - Narrative (Details) ft² in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Occupancy Expense and Information Systems and Communications Expense [Abstract]
Depreciation	$ 472	$ 443	$ 417
Property Subject to or Available for Operating Lease [Line Items]
Capital lease obligation	194	231
Interest expense on capital lease	22	32	38
Accumulated amortization on capital lease	365	334
Sublease rental expense	194	190	204
Sublease revenue	4	$ 4	$ 6
Future minimum sublease rental revenue on capital lease	43
Future minimum sublease rental revenue on operating lease	$ 16
One Lincoln Street
Property Subject to or Available for Operating Lease [Line Items]
Leased area (in sqft) | ft²	1,025
Noncancelable capital leases expiration	20 years
Sublease building (in sqft) | ft²	127