Asset Quality - Credit Risk Profile Based on Payment Activity (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	$ 698	$ 693
Credit Cards [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	697	680
Credit Cards [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1	13
Consumer Other - Key Community Bank [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1,436	1,352
Consumer Other - Key Community Bank [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1,434	1,349
Consumer Other - Key Community Bank [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	2	3
Consumer Marine [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	965	1,254
Consumer Marine [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	950	1,229
Consumer Marine [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	15	25
Consumer Others [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	63	79
Consumer Others [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	62	78
Consumer Others [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1	1
Consumer Loans [Member] | Credit Risk Profile [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	3,162	3,378
Consumer Loans [Member] | Performing [Member] | Credit Risk Profile [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	3,143	3,336
Consumer Loans [Member] | Nonperforming [Member] | Credit Risk Profile [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	$ 19	$ 42