Acquisitions and Discontinued Operations - Principal and Fair Value Amounts for Trust Loans at Fair Value, Portfolio Loans at Fair Value, and Portfolio Loans at Carrying Value (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accruing loans past due 90 days or more	$ 111	$ 96	$ 89
Trust [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			25
Loans placed on nonaccrual status			10
Accruing loans past due 90 days or more			25
Loans placed on nonaccrual status			10
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	7
Accruing loans past due 90 days or more	5	5	7
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	27	29	32
Loans placed on nonaccrual status	$ 8	$ 11	$ 8