Goodwill and Other Intangible Assets - Gross Carrying Amount and Accumulated Amortization of Intangible Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Gross Carrying Amount	$ 741	$ 687
Accumulated Amortization	325	303
Austin Capital Management [Member]
Goodwill [Line Items]
Gross Carrying Amount		18
Accumulated Amortization		18
Core Deposit Intangibles [Member]
Goodwill [Line Items]
Gross Carrying Amount	461	461
Accumulated Amortization	192	125
Purchased Credit Card Relationship Intangible [Member]
Goodwill [Line Items]
Gross Carrying Amount	152	152
Accumulated Amortization	126	110
Other Intangible Assets [Member]
Goodwill [Line Items]
Gross Carrying Amount	128	74
Accumulated Amortization	$ 7	$ 68