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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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