Loans and Loans Held for Sale - Loans by Category (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial, financial and agricultural	$ 31,095	$ 27,982	$ 26,683
Commercial real estate:
Commercial mortgage	8,180	8,047	8,276
Construction	1,070	1,100	1,036
Total commercial real estate loans	9,250	9,147	9,312
Commercial lease financing	3,929	4,252	4,135
Total commercial loans	44,274	41,381	40,130
Residential - prime loans:
Real estate - residential mortgage	2,267	2,225	2,213
Home equity loans	10,504	10,633	10,663
Total residential - prime loans	12,771	12,858	12,876
Credit cards	770	754	724
Consumer other	658	828	879
Total consumer loans	15,811	16,000	16,025
Loans Outstanding	60,085	57,381	56,155
Key Community Bank [Member]
Residential - prime loans:
Home equity loans	10,282	10,366	10,380
Consumer other	1,612	1,560	1,546
Other loans [Member]
Residential - prime loans:
Home equity loans	222	267	283
Consumer other	38	49	51
Marine loans [Member]
Residential - prime loans:
Consumer other	$ 620	$ 779	$ 828