Commitments (Other Commitments) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other Commitments [Line Items]
Commitments	$ 172,521	$ 163,897
Commitments to extend credit
Other Commitments [Line Items]
Commitments	159,641	151,981
Commitments to extend credit | Total commercial lending
Other Commitments [Line Items]
Commitments	112,125	108,256
Commitments to extend credit | Home Equity Line of Credit
Other Commitments [Line Items]
Commitments	17,852	17,438
Commitments to extend credit | Credit Card
Other Commitments [Line Items]
Commitments	24,911	22,095
Commitments to extend credit | Other
Other Commitments [Line Items]
Commitments	4,753	4,192
Standby letters of credit
Other Commitments [Line Items]
Commitments	8,651	8,324
Standby letters of credit | Remarketing Programs
Other Commitments [Line Items]
Commitments	3,500	3,900
Reinsurance Agreements
Other Commitments [Line Items]
Commitments	1,654	1,835
Reinsurance Agreements | Accidental Death and Dismemberment
Other Commitments [Line Items]
Maximum Exposure	1,500	1,500
Reinsurance Agreements | Credit Life Accident and Health
Other Commitments [Line Items]
Maximum Exposure	200	300
Standby bond purchase agreements
Other Commitments [Line Items]
Commitments	843	790
Other commitments
Other Commitments [Line Items]
Commitments	1,732	967
Other commitments | Investments in qualified affordable housing projects
Other Commitments [Line Items]
Commitments	$ 500	$ 500