Acquisitions and Discontinued Operations - Combined Assets and Liabilities of 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]
Cash and due from banks	$ 470,000,000	$ 653,000,000	$ 651,000,000	$ 617,000,000
Held-to-maturity securities	4,936,000,000	5,015,000,000	4,997,000,000
Portfolio loans at fair value		191,000,000	201,000,000
Loans, net of unearned income	60,085,000,000	57,381,000,000	56,155,000,000
Portfolio loans held for sale at fair value	0	0	0
Total assets	2,086,000,000	2,324,000,000	2,421,000,000
Total liabilities	0	3,000,000	3,000,000
Portfolio [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio loans held for sale at fair value	169,000,000
Trust [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio loans held for sale at fair value	173,000,000
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
Held-to-maturity securities	1,000,000	1,000,000	1,000,000
Loans, net of unearned income	1,891,000,000	2,104,000,000	2,174,000,000
Less: Allowance for loan and lease losses	23,000,000	29,000,000	31,000,000
Net loans	1,868,000,000	2,266,000,000	2,344,000,000
Accrued income and other assets	33,000,000	38,000,000	40,000,000
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] | Seller Note [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio loans at fair value			17,000,000
Discontinued Operations [Member] | Portfolio [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio loans at fair value		191,000,000	201,000,000
Portfolio loans held for sale at fair value	$ 169,000,000
Discontinued Operations [Member] | Trust [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accrued expense and other liabilities		$ 3,000,000	$ 3,000,000