Goodwill and Other Intangible Assets - Gross Carrying Amount and Accumulated Amortization of Intangible Assets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Goodwill [Line Items]
Gross Carrying Amount	$ 389	$ 376
Accumulated Amortization	288	249
Austin Capital Management [Member]
Goodwill [Line Items]
Gross Carrying Amount	18	18
Accumulated Amortization	18	18
Core Deposit Intangibles [Member]
Goodwill [Line Items]
Gross Carrying Amount	105	105
Accumulated Amortization	82	70
Purchased Credit Card Relationship Intangible [Member]
Goodwill [Line Items]
Gross Carrying Amount	136	136
Accumulated Amortization	69	44
Other Intangible Assets [Member]
Goodwill [Line Items]
Gross Carrying Amount	148	135
Accumulated Amortization	$ 137	$ 135