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Fair Value Measurements (Fair Value By Level) (Details) - USD ($) | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Assets: |  |
Available for sale debt securities | $ 4,691,938,000 | $ 4,417,796,000
Equity Securities | 5,158,000 | 5,358,000
Estimate of Fair Value Measurement |  |
Assets: |  |
Available for sale debt securities | 4,691,938,000 | 4,417,796,000
Estimate of Fair Value Measurement | US Treasury Securities |  |
Assets: |  |
Available for sale debt securities |  | 49,393,000
Estimate of Fair Value Measurement | Residential mortgage-backed securities |  |
Assets: |  |
Available for sale debt securities | 4,545,600,000 | 4,209,212,000
Estimate of Fair Value Measurement | States and political subdivisions |  |
Assets: |  |
Available for sale debt securities | 146,338,000 | 159,191,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement |  |
Assets: |  |
Equity Securities | 5,158,000 | 5,358,000
Marketable Securities | 4,697,096,000 | 4,423,154,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement | US Treasury Securities |  |
Assets: |  |
Available for sale debt securities |  | 49,393,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement | Residential mortgage-backed securities |  |
Assets: |  |
Available for sale debt securities | 4,545,600,000 | 4,209,212,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement | States and political subdivisions |  |
Assets: |  |
Available for sale debt securities | 146,338,000 | 159,191,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 1 |  |
Assets: |  |
Equity Securities | 5,158,000 | 5,358,000
Marketable Securities | 5,158,000 | 5,358,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 2 |  |
Assets: |  |
Marketable Securities | 4,691,938,000 | 4,417,796,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 2 | US Treasury Securities |  |
Assets: |  |
Available for sale debt securities |  | 49,393,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 2 | Residential mortgage-backed securities |  |
Assets: |  |
Available for sale debt securities | 4,545,600,000 | 4,209,212,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 2 | States and political subdivisions |  |
Assets: |  |
Available for sale debt securities | $ 146,338,000 | $ 159,191,000

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