Asset Quality - Nonperforming Assets and Past Due Loans (Detail) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total nonperforming loans	$ 723	$ 400	$ 387
OREO	35	17	14
Other nonperforming assets	2	0	2
Total nonperforming assets	760	417	403
Nonperforming assets from discontinued operations-education lending	5	8	7
Restructured loans included in nonperforming loans	149	159	159
Restructured loans with an allocated specific allowance	68	71	69
Specifically allocated allowance for restructured loans	38	29	30
Accruing loans past due 90 days or more	49	54	72
Accruing loans past due 30 through 89 days	317	271	208
Purchased credit impaired loans	959	12	11
Restructured loans	22	20	21
Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Carrying value of foreclosed residential real estate	175	114	114
Foreclosed Residential Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Carrying value of foreclosed residential real estate	$ 27	$ 13	$ 11