Asset Quality (Details 7) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Mar. 31, 2011
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	$ 49,226	$ 48,552
Consumer Loans [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	14,604	15,254
Consumer Other - Key Community Bank [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	1,212	1,141
Consumer Marine [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	1,654	2,112
Consumer Others [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	111	150
Performing [Member] | Consumer Other - Key Community Bank [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	1,211	1,138
Performing [Member] | Consumer Marine [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	1,624	2,081
Performing [Member] | Consumer Others [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	110	149
Nonperforming [Member] | Consumer Other - Key Community Bank [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	1	3
Nonperforming [Member] | Consumer Marine [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	30	31
Nonperforming [Member] | Consumer Others [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	1	1
Credit Risk Profile [Member] | Consumer Loans [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	2,977	3,403
Credit Risk Profile [Member] | Performing [Member] | Consumer Loans [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	2,945	3,368
Credit Risk Profile [Member] | Nonperforming [Member] | Consumer Loans [Member]
Credit Risk Profile Based on Payment Activity
Credit risk profile based on payment	$ 32	$ 35