Acquisition, Divestiture, and Discontinued Operations - Components of Assets and Liabilities from Discontinued Operations (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and due from banks	$ 677	$ 607	$ 653	$ 617
Discontinued assets	1,585	1,846
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and due from banks	15	15
Discontinued assets	1,585	1,846
Discontinued Operations [Member] | Austin Capital Management [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and due from banks	15	15
Discontinued assets	$ 15	$ 15