Basis of Presentation and Accounting Policies Basis of Presentation and Accounting Policies (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Jan. 01, 2018
Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Cumulative effect of changes to retained earnings as a result of adoption of ASU $ 8,000 $ 7,930 $ 7,963   $ 7,796
Network costs to be presented net of card fee revenue 59 [1] 77      
Proceeds from settlement of BOLI policies 3 3      
Remeasurement of the Corporation's deferred tax balance 104   $ 107    
Deferred tax balance downward revision to the estimated impact 3        
Accounting Standards Codification (ASC) Topic 606          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Decrease in card fees due to change in presentation 34        
Decrease in service charges on deposit accounts due to change in presentation 1        
Decrease in processing fee expense due to change in presentation 35        
Treasury management fee transition adjustment to retained earnings       $ 14  
Cumulative effect of changes to retained earnings as a result of adoption of ASU       $ 2  
Network costs to be presented net of card fee revenue 34 $ 25      
Accounting Standards Update 2016-15          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Proceeds from settlement of BOLI policies 3        
Accounting Standards Update 2017-07          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Other components of net benefit cost to be reclassified 12        
Accounting Standards Update 2017-12          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Cumulative effect of change to retained earnings as a result of adoption of ASU 1        
Recognition of additional unrealized losses from the transfer of Held-to-maturity to Available-for-sale securities $ 11        
[1] Adoption of Topic 606 resulted in a change in presentation which records certain costs in the same category as the associated revenues. The effect of this change was to reduce card fees by $34 million and service charges on deposit accounts by $1 million for the first quarter 2018. Refer to note 1 for further information.