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Investment Securities (Narrative) (Details)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
USD ($)
Segment
Sep. 30, 2020
USD ($)
Sep. 30, 2021
USD ($)
Segment
Sep. 30, 2020
USD ($)
Dec. 31, 2020
USD ($)
Security
Schedule Of Investments [Line Items]          
Securities pledged as collateral $ 861,200,000   $ 861,200,000   $ 567,400,000
Number of security positions, unrealized loss position 99   99   8
Number of security positions, unrealized loss position for more than 12 months 6   6   1
Securities, 12 months or longer, Fair Value $ 16,849,000   $ 16,849,000   $ 8,000
Securities, 12 months or longer, Unrealized Losses $ 541,000   $ 541,000    
Number of security positions, unrealized loss position for less than 12 months 93   93   7
Securities, Less than 12 months, Fair Value $ 768,267,000   $ 768,267,000   $ 29,087,000
Securities, Less than 12 months, Unrealized Losses 11,822,000   11,822,000   68,000
Available for sale securities, allowance for credit loss 0   0   0
Debt securities, held-to-maturity 218,140,000   218,140,000   271,973,000
Investment securities, past due 3,609,791,000   3,609,791,000   3,557,908,000
Investment securities, nonaccrual 6,675,000   6,675,000   9,286,000
Total Past Due [Member]          
Schedule Of Investments [Line Items]          
Investment securities, past due 17,947,000   17,947,000   7,994,000
Municipal Bonds [Member]          
Schedule Of Investments [Line Items]          
Debt securities, held-to-maturity 111,000,000.0   111,000,000.0   135,700,000
Internally rated held-to-maturity securities 5,800,000   5,800,000   8,500,000
Municipal Bonds [Member] | Below Investment Grade [Member]          
Schedule Of Investments [Line Items]          
Investment grade rating exposure         $ 279,000
Investment grade rating exposure percentage         0.19%
Maximum [Member]          
Schedule Of Investments [Line Items]          
Securities, 12 months or longer, Unrealized Losses         $ 1,000
Available for Sale Securities [Member]          
Schedule Of Investments [Line Items]          
Accrued interest receivable 1,900,000   1,900,000   1,200,000
Held To Maturity Investment Securities [Member]          
Schedule Of Investments [Line Items]          
Accrued interest receivable 1,000,000.0   1,000,000.0   905,000
Investment securities, nonaccrual 0   0   0
Held To Maturity Investment Securities [Member] | Total Past Due [Member]          
Schedule Of Investments [Line Items]          
Investment securities, past due 0   0   $ 0
Held To Maturity Investment Securities [Member] | Maximum [Member]          
Schedule Of Investments [Line Items]          
Credit loss expense $ (1,000) $ 0 $ (2,000) $ (6,000)