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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) |

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