Acquisitions and Discontinued Operations - Components of Assets and Liabilities from Discontinued Operations (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Seller note		$ 191,000,000	$ 201,000,000
Cash and due from banks	$ 470,000,000	653,000,000	651,000,000	$ 617,000,000
Total assets	2,086,000,000	2,324,000,000	2,421,000,000
Total liabilities	0	3,000,000	3,000,000
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and due from banks	15,000,000	19,000,000	19,000,000
Total assets	2,086,000,000	2,324,000,000	2,421,000,000
Total liabilities		3,000,000	3,000,000
Discontinued Operations [Member] | Victory Capital Management [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assets			17,000,000
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	15,000,000	19,000,000	19,000,000
Total assets	$ 15,000,000	19,000,000	19,000,000
Accrued expense and other liabilities		3,000,000	3,000,000
Total liabilities		$ 3,000,000	3,000,000
Discontinued Operations [Member] | Seller Note [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Seller note			17,000,000
Discontinued Operations [Member] | Seller Note [Member] | Victory Capital Management [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Seller note			$ 17,000,000