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Allowance for Credit Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Details) $ in Thousands | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2021 USD ($) SecurityLoan | Dec. 31, 2020 USD ($) SecurityLoan | Dec. 31, 2019 USD ($)
Financing Receivable, Past Due [Line Items] |  |  |
Total | $ 6,970,087 | $ 7,225,860 |
Non - performing | 111,312 | 146,774 | $ 54,057
Current |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 6,854,864 | 7,069,730 |
Total Past Due Loans |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 115,223 | 156,130 |
30-59 Days Past Due |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 2,022 | 2,888 |
60-89 Days Past Due |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 1,889 | 5,965 |
Past Due 90+ Days Still Accruing |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total |  | 503 |
Commercial |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 5,423,964 | $ 5,673,707 |
Non - performing | $ 1,700 |  |
Commercial | CARES Act, Paycheck Protection Program Loans |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Number of modified loans | SecurityLoan | 42 | 52 |
Modified loans | $ 58,900 | $ 195,600 |
Commercial | Commercial real estate |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 2,702,780 | 2,791,947 |
Non - performing | 54,797 | 101,070 | 25,356
Commercial | Commercial and industrial |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 1,478,198 | 1,559,552 |
Non - performing | 33,909 | 16,985 | 10,911
Commercial | Commercial construction |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 485,924 | 466,077 |
Non - performing | 4,053 | 384 | 737
Commercial | Business banking |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 1,100,125 | 1,160,067 |
Non - performing | 10,221 | 17,122 | 9,863
Commercial | Current | Commercial real estate |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 2,647,983 | 2,690,877 |
Commercial | Current | Commercial and industrial |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 1,444,289 | 1,542,567 |
Commercial | Current | Commercial construction |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 481,871 | 462,094 |
Commercial | Current | Business banking |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 1,088,013 | 1,140,581 |
Commercial | Total Past Due Loans | Commercial real estate |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 54,797 | 101,070 |
Commercial | Total Past Due Loans | Commercial and industrial |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 33,909 | 16,985 |
Commercial | Total Past Due Loans | Commercial construction |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 4,053 | 3,983 |
Commercial | Total Past Due Loans | Business banking |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 12,112 | 19,486 |
Commercial | 30-59 Days Past Due | Commercial real estate |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 0 | 0 |
Commercial | 30-59 Days Past Due | Commercial and industrial |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 0 | 0 |
Commercial | 30-59 Days Past Due | Commercial construction |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 0 | 19 |
Commercial | 30-59 Days Past Due | Business banking |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 970 | 1,614 |
Commercial | 60-89 Days Past Due | Commercial real estate |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 0 | 0 |
Commercial | 60-89 Days Past Due | Commercial and industrial |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 0 | 0 |
Commercial | 60-89 Days Past Due | Commercial construction |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 0 | 3,580 |
Commercial | 60-89 Days Past Due | Business banking |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 921 | 379 |
Commercial | Past Due 90+ Days Still Accruing | Commercial real estate |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total |  | 0 |
Commercial | Past Due 90+ Days Still Accruing | Commercial and industrial |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total |  | 0 |
Commercial | Past Due 90+ Days Still Accruing | Commercial construction |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total |  | 0 |
Commercial | Past Due 90+ Days Still Accruing | Business banking |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total |  | 371 |
Consumer |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 1,546,123 | 1,552,153 |
Consumer | Business banking |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total |  | 303,900 |
Consumer | Consumer real estate |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 1,105,478 | 1,167,332 |
Non - performing | 8,143 | 11,117 | 6,063
Consumer | Other consumer |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 97,582 | 80,885 |
Non - performing | 189 | 96 | $ 1,127
Consumer | Current | Consumer real estate |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 1,095,557 | 1,153,028 |
Consumer | Current | Other consumer |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 97,151 | 80,583 |
Consumer | Total Past Due Loans | Consumer real estate |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 9,921 | 14,304 |
Consumer | Total Past Due Loans | Other consumer |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 431 | 302 |
Consumer | 30-59 Days Past Due | Consumer real estate |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 833 | 1,087 |
Consumer | 30-59 Days Past Due | Other consumer |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 219 | 168 |
Consumer | 60-89 Days Past Due | Consumer real estate |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | 945 | 1,968 |
Consumer | 60-89 Days Past Due | Other consumer |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total | $ 23 | 37 |
Consumer | Past Due 90+ Days Still Accruing | Consumer real estate |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total |  | 132 |
Consumer | Past Due 90+ Days Still Accruing | Other consumer |  |  |
Financing Receivable, Past Due [Line Items] |  |  |
Total |  | $ 0 |

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