Acquisitions and Discontinued Operations - Principal and Fair Value Amounts for Portfolio Loans Held for Sale at Fair Value, Portfolio Loans at Carrying Value, and Portfolio Loans at Fair Value (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accruing loans past due 90 days or more	$ 54	$ 96	$ 71
Portfolio Loans Held For Sale [Member] | Fair Value [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accruing loans past due 90 days or more	5
Accruing loans past due 90 days or more	4
Portfolio [Member] | Fair Value [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accruing loans past due 90 days or more		5	5
Accruing loans past due 90 days or more		5	5
Portfolio [Member] | Carrying Amount [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accruing loans past due 90 days or more	26	29	31
Loans placed on nonaccrual status	$ 8	$ 11	$ 9