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Fair Value (Fair Value By Level) (Details) - USD ($) | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Assets: |  |
Available for sale debt securities | $ 3,080,768,000 | $ 3,378,923,000
Equity Securities | 6,202,000 | 6,095,000
Marketable Securities | 3,090,370,000 | 3,387,418,000
Estimate of Fair Value Measurement [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 3,029,954,000 | 3,285,548,000
Estimate of Fair Value Measurement [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 50,814,000 | 93,375,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Assets: |  |
Equity Securities | 6,202,000 | 6,095,000
Marketable Securities | 6,202,000 | 6,095,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Assets: |  |
Marketable Securities | 3,080,768,000 | 3,378,923,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 3,029,954,000 | 3,285,548,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 50,814,000 | 93,375,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] |  |
Assets: |  |
Equity Securities | 6,202,000 | 6,095,000
Marketable Securities | 3,086,970,000 | 3,385,018,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Assets: |  |
Available for sale debt securities | 3,029,954,000 | 3,285,548,000
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 | $ 50,814,000 | $ 93,375,000

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