Acquisition, Divestiture, and Discontinued Operations - Components of Assets of Education Lending Business (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio loans at fair value	$ 2
Loans, net of unearned income	86,492	$ 86,038	$ 85,528
Total assets	1,396	1,585
Unearned income on loans (less than)	734	826
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio loans at fair value	2	3
Loans, net of unearned income	1,370	1,562
Less: Allowance for loan and lease losses	18	24
Net loans	1,354	1,541
Accrued income and other assets	26	28
Total assets	1,396	1,585
Unearned income on loans (less than)	1	1
Education Lending [Member] | Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Held-to-maturity securities	1	1
Portfolio loans at fair value	2	3
Loans, net of unearned income	1,370	1,562
Less: Allowance for loan and lease losses	18	24
Net loans	1,354	1,541
Accrued income and other assets	26	28
Total assets	1,381	1,570
Unearned income on loans (less than)	$ 1	$ 1