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Summary of Significant Accounting Policies - Premises and Equipment (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2016
Jan. 17, 2017
Dec. 31, 2015
Transfers and Servicing of Financial Assets      
Mortgage loans serviced for others $ 668.2   $ 638.2
Subsequent Events      
Cash dividend declared (per share)   $ 1.00  
Buildings | Minimum      
Premises and equipment      
Useful lives 25 years    
Buildings | Maximum      
Premises and equipment      
Useful lives 40 years    
Building improvements | Minimum      
Premises and equipment      
Useful lives 3 years    
Building improvements | Maximum      
Premises and equipment      
Useful lives 15 years    
Furniture and equipment | Minimum      
Premises and equipment      
Useful lives 3 years    
Furniture and equipment | Maximum      
Premises and equipment      
Useful lives 10 years