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Fair Value (Fair Value By Level) (Details) - USD ($) | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Assets: |  |
Available for sale debt securities | $ 4,057,357,000 | $ 3,080,768,000
Equity Securities | 6,125,000 | 6,202,000
Marketable Securities | 4,066,882,000 | 3,090,370,000
Estimate of Fair Value Measurement [Member] | US Treasury Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 945,682,000 |
Estimate of Fair Value Measurement [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 3,063,036,000 | 3,029,954,000
Estimate of Fair Value Measurement [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 48,639,000 | 50,814,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] |  |
Assets: |  |
Marketable Securities | 4,063,482,000 | 3,086,970,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | US Treasury Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 945,682,000 |
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 3,063,036,000 | 3,029,954,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 48,639,000 | 50,814,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Equity Securities [Member] |  |
Assets: |  |
Equity Securities | 6,125,000 | 6,202,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Assets: |  |
Marketable Securities | 951,807,000 | 6,202,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 945,682,000 |
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] |  |
Assets: |  |
Equity Securities | 6,125,000 | 6,202,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Assets: |  |
Marketable Securities | 3,111,675,000 | 3,080,768,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 3,063,036,000 | 3,029,954,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Assets: |  |
Available for sale debt securities | $ 48,639,000 | $ 50,814,000

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