Asset Quality - Credit Risk Profile Based on Payment Activity (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Consumer Other - Key Community Bank [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	$ 1,440	$ 1,312
Consumer Other - Key Community Bank [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1,438	1,309
Consumer Other - Key Community Bank [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	2	3
Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	698	710
Credit Cards [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	694	702
Credit Cards [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	4	8
Consumer Marine [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1,083	1,448
Consumer Marine [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1,058	1,417
Consumer Marine [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	25	31
Consumer Others [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	71	98
Consumer Others [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	69	96
Consumer Others [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	2	2
Consumer Loans [Member] | Credit Risk Profile [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	3,292	3,568
Consumer Loans [Member] | Credit Risk Profile [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	3,259	3,524
Consumer Loans [Member] | Credit Risk Profile [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	$ 33	$ 44