Asset Quality - Credit Risk Profile Based on Payment Activity (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	$ 59,876	$ 57,381	$ 54,457
Consumer Other - Key Community Bank [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1,600	1,560
Consumer Other - Key Community Bank [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	1,598	1,558
Consumer Other - Key Community Bank [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	2	2
Consumer Credit Card Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	806	754
Consumer Credit Card Financing Receivable [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	804	752
Consumer Credit Card Financing Receivable [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	2	2
Consumer Marine [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	583	779
Consumer Marine [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	577	764
Consumer Marine [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	6	15
Consumer Others [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	38	49
Consumer Others [Member] | Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	38	48
Consumer Others [Member] | Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment		1
Consumer Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	15,604	16,000
Consumer Loans [Member] | Credit Risk Profile [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	3,027	3,142
Consumer Loans [Member] | Performing [Member] | Credit Risk Profile [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	3,017	3,122
Consumer Loans [Member] | Nonperforming [Member] | Credit Risk Profile [Member]
Financing Receivable, Recorded Investment [Line Items]
Credit risk profile based on payment	$ 10	$ 20