Asset Quality - Credit Risk Profile Based on Payment Activity (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	$ 60,085	$ 57,381	$ 56,155
Consumer Other - Key Community Bank [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1,612	1,560	1,546
Consumer Other - Key Community Bank [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1,611		1,544
Consumer Other - Key Community Bank [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1		2
Consumer Credit Card Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	770	754	724
Consumer Credit Card Financing Receivable [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	768		723
Consumer Credit Card Financing Receivable [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	2		1
Consumer Marine [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	620	779	828
Consumer Marine [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	614		812
Consumer Marine [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	6		16
Consumer Others [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	38	49	51
Consumer Others [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	37		50
Consumer Others [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1		1
Consumer Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	15,811	$ 16,000	16,025
Consumer Loans [Member] | Credit Risk Profile [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	3,040		3,149
Consumer Loans [Member] | Performing [Member] | Credit Risk Profile [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	3,030		3,129
Consumer Loans [Member] | Nonperforming [Member] | Credit Risk Profile [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	$ 10		$ 20