Goodwill and Other Intangible Assets - Gross Carrying Amount and Accumulated Amortization of Intangible Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Gross Carrying Amount	$ 687	$ 317
Accumulated Amortization	303	252
Austin Capital Management [Member]
Goodwill [Line Items]
Gross Carrying Amount	18	18
Core Deposit Intangibles [Member]
Goodwill [Line Items]
Gross Carrying Amount	461	105
Accumulated Amortization	125	91
Purchased Credit Card Relationship Intangible [Member]
Goodwill [Line Items]
Gross Carrying Amount	152	136
Accumulated Amortization	110	91
Other Intangible Assets [Member]
Goodwill [Line Items]
Gross Carrying Amount	74	76
Accumulated Amortization	$ 68	$ 70