Long-Term Debt - Components of Long-Term Debt (Detail) $ in Millions	Aug. 01, 2016 debenture	Dec. 31, 2016 USD ($)	May 20, 2016 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Total long-term debt		$ 12,384		$ 10,184
Real Estate Loans and securities pledged		126		251
Senior Medium-Term Notes Due Through 2021 [Member]
Debt Instrument [Line Items]
Senior medium-term notes		$ 2,799		$ 2,817
Long-term debt weighted average interest rate		3.57%		3.58%
7.25% Subordinated Notes Due 2021 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		7.25%
Subordinated long-term notes		$ 358		$ 0
6.75% Subordinated Notes Due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		6.75%
Subordinated long-term notes		$ 338		0
1.586% Subordinated Notes Due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		1.586%
Subordinated long-term notes		$ 162		162
6.875% Subordinated Notes Due 2029 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		6.875%
Subordinated long-term notes		$ 111		114
7.750% Subordinated Notes Due 2029 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		7.75%
Subordinated long-term notes		$ 141		147
Other subordinated notes [Member]
Debt Instrument [Line Items]
Subordinated long-term notes		78		0
Senior Medium-Term Notes Due Through 2039 [Member]
Debt Instrument [Line Items]
Senior medium-term notes		$ 6,715		$ 5,242
Long-term debt weighted average interest rate		1.93%		1.99%
3.18% Senior Remarketable Notes Due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		3.18%
Subordinated long-term notes		$ 193		$ 183
5.45% Subordinated Notes Due 2016 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		5.45%
Subordinated long-term notes		$ 0		503
5.70% Subordinated Notes Due 2017 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		5.70%
Subordinated long-term notes		$ 207		215
4.625% Subordinated Notes Due 2018 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		4.625%
Subordinated long-term notes		$ 102		103
3.40% Subordinated notes due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		3.40%	3.40%
Subordinated long-term notes		$ 567	$ 600	0
6.95% Subordinated Notes Due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		6.95%
Subordinated long-term notes		$ 299		298
Secured Notes Due Through 2021 [Member]
Debt Instrument [Line Items]
Secured borrowing due through 2021		$ 68		$ 134
Long-term debt weighted average interest rate		4.45%		4.42%
Federal Home Loan Bank [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank advances due through 2036		$ 126		$ 166
Long-term debt weighted average interest rate		3.64%		3.58%
Investment Fund Financing [Member]
Debt Instrument [Line Items]
Investment Fund Financing due through 2052		$ 100		$ 100
Long-term debt weighted average interest rate		1.94%		1.94%
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Obligations under Capital Leases due 2032		$ 20		$ 0
Key [Member]
Debt Instrument [Line Items]
Total long-term debt		3,987		3,240
Subsidiaries [Member]
Debt Instrument [Line Items]
Total long-term debt		$ 8,397		$ 6,944
Maximum [Member] | Other subordinated notes [Member]
Debt Instrument [Line Items]
Long-term debt weighted average interest rate		10.875%
Minimum [Member] | Other subordinated notes [Member]
Debt Instrument [Line Items]
Long-term debt weighted average interest rate		2.167%
First Niagara Bank, N.A. [Member] | Other subordinated notes [Member]
Debt Instrument [Line Items]
Number of trust preferred debentures acquired | debenture	7