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Summary of Significant Accounting Policies - Narrative (Details) | 12 Months Ended
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Dec. 31, 2021 USD ($) segment subsidiary | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Property, Plant and Equipment |  |  |
Benefit of stock-based compensation plans |  |  | $ (1,200,000)
Impairment losses | $ 0 | $ 0 | 0
Number of significant subsidiaries | subsidiary | 1 |  |
Income tax examination, penalties | $ 0 | 0 | 0
Cash on deposit with Federal Reserve Bank | 2,166,100,000 | 1,989,300,000 |
Compensating balances with Federal Reserve Bank | 0 |  |
Federal Reserve Bank stock | 42,800,000 |  |
Federal Home Loan Bank stock | 10,600,000 |  |
Advertising expense | 2,600,000 | 2,800,000 | $ 4,200,000
Loans and Leases Receivable Disclosure [Line Items] |  |  |
Accrued interest receivable | 47,400,000 | 51,100,000 |
Accrued Investment Income Receivable | 16,600,000 | 12,600,000 |
Hosting Arrangement, Service Contract, Implementation Cost, Expense, Amortization | $ 4,600,000 | 6,000,000 |
Number of operating segments | segment | 1 |  |
Buildings and improvements | Minimum |  |  |
Property, Plant and Equipment |  |  |
Estimated useful lives | 5 years |  |
Buildings and improvements | Maximum [Member] |  |  |
Property, Plant and Equipment |  |  |
Estimated useful lives | 45 years |  |
Furniture and equipment | Minimum |  |  |
Property, Plant and Equipment |  |  |
Estimated useful lives | 3 years |  |
Furniture and equipment | Maximum [Member] |  |  |
Property, Plant and Equipment |  |  |
Estimated useful lives | 15 years |  |
Loans |  |  |
Loans and Leases Receivable Disclosure [Line Items] |  |  |
Accrued interest receivable | $ 30,200,000 | 37,900,000 |
Common Stock Including Additional Paid in Capital [Member] |  |  |
Property, Plant and Equipment |  |  |
Benefit of stock-based compensation plans | $ (500,000) | $ (400,000) |

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