Long-Term Debt - Components of Long-Term Debt (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Total long-term debt	$ 7,875	$ 7,650
Senior Medium-Term Notes Due Through 2021 [Member]
Debt Instrument [Line Items]
Senior medium-term notes	2,575	2,553
0.975% Subordinated Notes Due 2028 [Member]
Debt Instrument [Line Items]
Subordinated long-term notes	162	162
6.875% Subordinated Notes Due 2029 [Member]
Debt Instrument [Line Items]
Subordinated long-term notes	113	103
7.750% Subordinated Notes Due 2029 [Member]
Debt Instrument [Line Items]
Subordinated long-term notes	147	133
Senior Medium-Term Notes Due Through 2039 [Member]
Debt Instrument [Line Items]
Senior medium-term notes	2,611	1,858
7.413% Subordinated Remarketable Notes Due 2027 [Member]
Debt Instrument [Line Items]
Subordinated long-term notes	272	270
5.80% Subordinated Notes Due 2014 [Member]
Debt Instrument [Line Items]
Subordinated long-term notes		768
4.95% Subordinated Notes Due 2015 [Member]
Debt Instrument [Line Items]
Subordinated long-term notes	251	251
5.45% Subordinated Notes Due 2016 [Member]
Debt Instrument [Line Items]
Subordinated long-term notes	524	544
5.70% Subordinated Notes Due 2017 [Member]
Debt Instrument [Line Items]
Subordinated long-term notes	222	229
4.625% Subordinated Notes Due 2018 [Member]
Debt Instrument [Line Items]
Subordinated long-term notes	103	104
6.95% Subordinated Notes Due 2028 [Member]
Debt Instrument [Line Items]
Subordinated long-term notes	298	298
Lease Financing Debt Due Through 2016 [Member]
Debt Instrument [Line Items]
Lease financing debt due through 2016		5
Secured Notes Due Through 2020 [Member]
Debt Instrument [Line Items]
Secured borrowings due through 2020	302	58
Federal Home Loan Bank [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank advances due through 2036	200	224
Investment Fund Financing [Member]
Debt Instrument [Line Items]
Investment Fund Financing due through 2052	95	90
Key [Member]
Debt Instrument [Line Items]
Total long-term debt	2,997	2,951
Subsidiaries [Member]
Debt Instrument [Line Items]
Total long-term debt	$ 4,878	$ 4,699