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Securities Available for Sale and Held to Maturity - Additional Information (Details) - USD ($)
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Jan. 01, 2020
Dec. 31, 2019
Dec. 31, 2013
Schedule of Available For Sale and Held to Maturity Securities [Line Items]          
Retained earnings $ 1,459,306,000     $ 1,414,526,000  
Securities held to maturity 611,280,000     738,099,000  
Potential credit loss exposure 31,500,000        
Reclassification of Securities available for sale to securities held to maturity         $ 1,099,000,000.000
Net unrealized holding loss on AFS Securities at date of transfer         46,600,000
Net unrealized holding losses on AFS Securities, net of tax at date of transfer         $ 28,800,000
Net unamortized, unrealized loss on transfer of securities 9,700,000     12,100,000  
Net unamortized, unrealized loss on transfer of securities, net of tax 7,300,000     9,100,000  
Other-than-temporary impairments   $ 0      
Pledged to collateralize public deposits and securities sold under repurchase agreements and for other purposes as permitted by law 1,991,000,000     1,770,000,000  
Pledged securities providing additional contingency funding 0     $ 0  
30 Days or More Past Due [Member]          
Schedule of Available For Sale and Held to Maturity Securities [Line Items]          
Held-to-maturity, past due 0        
Securities Available for Sale [Member]          
Schedule of Available For Sale and Held to Maturity Securities [Line Items]          
Accrued interest receivable 4,200,000        
Securities Held to Maturity [Member]          
Schedule of Available For Sale and Held to Maturity Securities [Line Items]          
Accrued interest receivable 1,500,000        
Reserve for credit loss 0        
Held-to-maturity nonnaccrual 0        
ASU 2016-13 [Member]          
Schedule of Available For Sale and Held to Maturity Securities [Line Items]          
Securities held to maturity 611,300,000        
ASU 2016-13 [Member] | Securities Available for Sale [Member]          
Schedule of Available For Sale and Held to Maturity Securities [Line Items]          
Expected credit loss 0        
ASU 2016-13 [Member] | Securities Available for Sale [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]          
Schedule of Available For Sale and Held to Maturity Securities [Line Items]          
Retained earnings     $ 0    
ASU 2016-13 [Member] | Securities Held to Maturity [Member]          
Schedule of Available For Sale and Held to Maturity Securities [Line Items]          
Current expected credit loss $ 0        
ASU 2016-13 [Member] | Securities Held to Maturity [Member] | Cumulative Effect Period Of Adoption Adjustment [Member]          
Schedule of Available For Sale and Held to Maturity Securities [Line Items]          
Retained earnings     $ 0