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Fair Value Measurements - Fair Value By Level (Details) - USD ($) | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Assets: |  |
Available for sale debt securities | $ 4,769,630,000 | $ 4,822,341,000
Equity Securities | 5,360,000 | 5,417,000
Estimate of Fair Value Measurement |  |
Assets: |  |
Available for sale debt securities | 4,769,630,000 | 4,822,341,000
Estimate of Fair Value Measurement | Residential mortgage-backed securities |  |
Assets: |  |
Available for sale debt securities | 4,612,306,000 | 4,660,099,000
Estimate of Fair Value Measurement | States and political subdivisions |  |
Assets: |  |
Available for sale debt securities | 157,324,000 | 162,242,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement |  |
Assets: |  |
Equity Securities | 5,360,000 | 5,417,000
Marketable Securities | 4,774,990,000 | 4,827,758,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement | Residential mortgage-backed securities |  |
Assets: |  |
Available for sale debt securities | 4,612,306,000 | 4,660,099,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement | States and political subdivisions |  |
Assets: |  |
Available for sale debt securities | 157,324,000 | 162,242,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 1 |  |
Assets: |  |
Equity Securities | 5,360,000 | 5,417,000
Marketable Securities | 5,360,000 | 5,417,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 2 |  |
Assets: |  |
Marketable Securities | 4,769,630,000 | 4,822,341,000
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 | 4,612,306,000 | 4,660,099,000
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 | $ 157,324,000 | $ 162,242,000

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