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Summary of Significant Accounting Policies Significant Accounting Policies (Narrative) (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
USD ($)
Locations
Jan. 01, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Accounting Policies [Abstract]          
Locations | Locations 300        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Retained Earnings $ 3,399,972   $ 3,138,472 $ 2,937,149 $ 2,718,184
Allowance for Credit Loss 220,834 $ 139,643      
Off-Balance Sheet, Credit Loss, Liability 38,307 17,165 1,075    
Restricted Cash and Cash Equivalents 23,400   20,300    
Cash Held at Federal Reserve Bank $ 1,700,000        
Amortization period of annual fees on credit card loans, months 12 months        
Consumer Loans Charged Down to Fair Value [Line Items]          
Delinquency Period Revolving Home Equity Loans Charged Down to Fair Value 180 days        
Period past due credit card loans are charged off, days 180 days        
Period past due loans are placed on non-accrual, days 90 days        
Finite-Lived Intangible Assets [Line Items]          
Common stock dividend rate percentage 5.00%        
Commercial Portfolio Segment [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Allowance for Credit Loss $ 121,549 62,049      
Off-Balance Sheet, Credit Loss, Liability 37,259 16,456      
Personal Banking Portfolio Segment [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Allowance for Credit Loss 99,285 77,594      
Off-Balance Sheet, Credit Loss, Liability 1,048 709      
Retained Earnings          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Retained Earnings $ 73,000   $ 201,562 $ 241,163 $ 221,374
Minimum [Member]          
Consumer Loans Charged Down to Fair Value [Line Items]          
Consumer loans charged down to fair value, days delinquent 120 days        
Maximum [Member]          
Consumer Loans Charged Down to Fair Value [Line Items]          
Consumer loans charged down to fair value, days delinquent 180 days        
Buildings [Member]          
Property, Plant and Equipment [Line Items]          
Depreciable lives for asset, years 30 years        
Building Improvements [Member]          
Property, Plant and Equipment [Line Items]          
Depreciable lives for asset, years 10 years        
Equipment [Member] | Minimum [Member]          
Property, Plant and Equipment [Line Items]          
Depreciable lives for asset, years 3 years        
Equipment [Member] | Maximum [Member]          
Property, Plant and Equipment [Line Items]          
Depreciable lives for asset, years 10 years        
Leasehold Improvements [Member]          
Property, Plant and Equipment [Line Items]          
Depreciable lives for asset, years 10 years        
Core Deposits [Member] | Minimum [Member]          
Finite-Lived Intangible Assets [Line Items]          
Finite-Lived Intangible Asset, Useful Life 8 years        
Core Deposits [Member] | Maximum [Member]          
Finite-Lived Intangible Assets [Line Items]          
Finite-Lived Intangible Asset, Useful Life 14 years        
Accounting Standards Update 2016-13          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Allowance for Credit Loss   (21,039)      
Off-Balance Sheet, Credit Loss, Liability   16,090      
Deferred Income Tax Assets, Net   (1,200)      
Accounting Standards Update 2016-13 | Commercial Portfolio Segment [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Allowance for Credit Loss   (29,711)      
Accounting Standards Update 2016-13 | Personal Banking Portfolio Segment [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Allowance for Credit Loss   8,672      
Accounting Standards Update 2016-13 | Retained Earnings          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Retained Earnings   $ 3,800