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Fair Market Value of Financial Assets and Liabilities, Accompanying Balance Sheets Measured at Fair Value (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2021 | Jun. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | $ 1,525,738 |  |  | $ 1,525,738 |  | $ 997,261
Equity securities at fair value | 2,461 |  |  | 2,461 |  | 2,471
Assets measured-nonrecurring basis [Abstract] |  |  |  |  |  |
Collateral dependent loans | 1,264 |  |  | 1,264 |  | 1,768
Reconciliation of beginning and ending balances of recurring fair value measurements recognized in balance sheet using significant unobservable (Level 3) inputs [Roll Forward] |  |  |  |  |  |
Total gains (losses) | (103) |  | $ 99 | $ 370 | $ (1,066) |
Minimum [Member] |  |  |  |  |  |
Other real estate owned [Abstract] |  |  |  |  |  |
Typical frequency of periodic reviews |  |  |  | 12 months |  |
Maximum [Member] |  |  |  |  |  |
Other real estate owned [Abstract] |  |  |  |  |  |
Typical frequency of periodic reviews |  |  |  | 18 months |  |
Frequency of periodic reviews in general |  |  |  | 24 months |  |
U.S. Treasury and Government Agencies [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 302,410 |  |  | $ 302,410 |  | 148,793
State and Political Subdivisions [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 335,556 |  |  | 335,556 |  | 140,416
U.S. Government Sponsored Agency Mortgage-backed Securities [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 787,507 |  |  | 787,507 |  | 651,807
Asset-backed Securities [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 100,265 |  |  | 100,265 |  | 56,245
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 244,361 |  |  | 244,361 |  | 74,991
Equity securities at fair value | 0 |  |  | 0 |  | 0
Significant Other Observable Inputs (Level 2) [Member] | Fair Value [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 1,281,377 |  |  | 1,281,377 |  | 922,270
Equity securities at fair value | 0 |  |  | 0 |  | 0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 0 |  |  | 0 |  | 0
Equity securities at fair value | 2,461 |  |  | 2,461 |  | 2,471
Recurring [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Equity securities at fair value | 2,461 |  |  | 2,461 |  | 2,471
Mortgage servicing rights | 6,265 |  |  | 6,265 |  | 4,068
Recurring [Member] | U.S. Treasury and Government Agencies [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 302,410 |  |  | 302,410 |  | 148,793
Recurring [Member] | State and Political Subdivisions [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 335,556 |  |  | 335,556 |  | 140,416
Recurring [Member] | U.S. Government Sponsored Agency Mortgage-backed Securities [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 787,507 |  |  | 787,507 |  | 651,807
Recurring [Member] | Asset-backed Securities [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 100,265 |  |  | 100,265 |  | 56,245
Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Equity securities at fair value | 0 |  |  | 0 |  | 0
Mortgage servicing rights | 0 |  |  | 0 |  | 0
Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Treasury and Government Agencies [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 244,361 |  |  | 244,361 |  | 74,991
Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | State and Political Subdivisions [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 0 |  |  | 0 |  | 0
Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Government Sponsored Agency Mortgage-backed Securities [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 0 |  |  | 0 |  | 0
Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Asset-backed Securities [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 0 |  |  | 0 |  | 0
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Equity securities at fair value | 0 |  |  | 0 |  | 0
Mortgage servicing rights | 0 |  |  | 0 |  | 0
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury and Government Agencies [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 58,049 |  |  | 58,049 |  | 73,802
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | State and Political Subdivisions [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 335,556 |  |  | 335,556 |  | 140,416
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | U.S. Government Sponsored Agency Mortgage-backed Securities [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 787,507 |  |  | 787,507 |  | 651,807
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Asset-backed Securities [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 100,265 |  |  | 100,265 |  | 56,245
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Equity securities at fair value | 2,461 |  |  | 2,461 |  | 2,471,000
Mortgage servicing rights | 6,265 |  |  | 6,265 |  | 4,068
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury and Government Agencies [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 0 |  |  | 0 |  | 0
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | State and Political Subdivisions [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 0 |  |  | 0 |  | 0
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Government Sponsored Agency Mortgage-backed Securities [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 0 |  |  | 0 |  | 0
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Asset-backed Securities [Member] |  |  |  |  |  |
Assets Measured at Fair Value on Recurring Basis [Abstract] |  |  |  |  |  |
Debt securities available-for-sale | 0 |  |  | 0 |  | 0
Recurring [Member] | Equity Securities [Member] |  |  |  |  |  |
Reconciliation of beginning and ending balances of recurring fair value measurements recognized in balance sheet using significant unobservable (Level 3) inputs [Roll Forward] |  |  |  |  |  |
Beginning balance | 2,523 |  | 2,094 | 2,471 | 1,953 | 1,953
Total unrealized gains (losses) included in net income | (62) |  | 118 | (10) | 259 |
Issues | 0 |  | 0 | 0 | 0 |
Settlements | 0 |  | 0 | 0 | 0 |
Ending balance | 2,461 | $ 2,523 | 2,212 | 2,461 | 2,212 | 2,471
Total gains (losses) for the period included in net income attributable to the change in unrealized gains or losses related to assets still held at the reporting date | (62) |  | 119 | (10) | 259 |
Recurring [Member] | Mortgage Servicing Rights [Member] |  |  |  |  |  |
Reconciliation of beginning and ending balances of recurring fair value measurements recognized in balance sheet using significant unobservable (Level 3) inputs [Roll Forward] |  |  |  |  |  |
Beginning balance | 5,899 |  | 2,518 | 4,068 | 3,263 | 3,263
Total unrealized gains (losses) included in net income | 141 |  | 286 | 1,042 | (680) |
Issues | 407 |  | 611 | 1,817 | 1,171 |
Settlements | (182) |  | (306) | (662) | (645) |
Ending balance | 6,265 | 5,899 | 3,109 | 6,265 | 3,109 | 4,068
Total gains (losses) for the period included in net income attributable to the change in unrealized gains or losses related to assets still held at the reporting date | 141 |  | 286 | 1,042 | (680) |
Nonrecurring [Member] |  |  |  |  |  |
Impaired loan (collateral dependent) [Abstract] |  |  |  |  |  |
Fair value adjustments on collateral dependent loans | 100 | 500 | 400 | 800 | 1,700 | 500
Other real estate owned [Abstract] |  |  |  |  |  |
Other real estate owned, fair value adjustment | 100 | $ 300 | $ 200 | 100 | $ 600 | 700
Nonrecurring [Member] | Fair Value [Member] |  |  |  |  |  |
Assets measured-nonrecurring basis [Abstract] |  |  |  |  |  |
Collateral dependent loans | 1,264 |  |  | 1,264 |  | 1,768
Other real estate owned | 522 |  |  | 522 |  | 2,395
Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value [Member] |  |  |  |  |  |
Assets measured-nonrecurring basis [Abstract] |  |  |  |  |  |
Collateral dependent loans | 0 |  |  | 0 |  | 0
Other real estate owned | 0 |  |  | 0 |  | 0
Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value [Member] |  |  |  |  |  |
Assets measured-nonrecurring basis [Abstract] |  |  |  |  |  |
Collateral dependent loans | 0 |  |  | 0 |  | 0
Other real estate owned | 0 |  |  | 0 |  | 0
Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value [Member] |  |  |  |  |  |
Assets measured-nonrecurring basis [Abstract] |  |  |  |  |  |
Collateral dependent loans | 1,264 |  |  | 1,264 |  | 1,768
Other real estate owned | $ 522 |  |  | $ 522 |  | $ 2,395

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