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Acquisitions Acquisitions (Narrative)(Details) - USD ($)
$ / shares in Units, $ in Millions
Jul. 24, 2018
Mar. 31, 2018
Apr. 01, 2017
Mar. 31, 2017
Oct. 01, 2016
Sep. 30, 2016
HIghland County Bankshares Inc and Subsidiary First and Citizens Bank [Member]            
Business Acquisition [Line Items]            
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets           $ 123
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities           $ 107
Business Acquisition, Percentage of Voting Interests Acquired         100.00%  
First Century Bankshares, Inc [Member]            
Business Acquisition [Line Items]            
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets       $ 406    
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities       $ 361    
Business Acquisition, Percentage of Voting Interests Acquired     100.00%      
Peoples Bankshares Inc [Member]            
Business Acquisition [Line Items]            
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets   $ 131        
Subsequent Event [Member] | Peoples Bankshares Inc [Member]            
Business Acquisition [Line Items]            
Business Acquisition, Share Price $ 47.00          
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares 1.7193          
Business Acquisition Combination of Equity and Cash, Cash 50.00%          
Business Acquisition Combination of Equity and Cash, Common Stock 50.00%