Fair Value Measurements (Fair Value By Level) (Details) - USD ($)	Jun. 30, 2022	Dec. 31, 2021
Assets:
Available for sale debt securities	$ 4,294,360,000	$ 4,213,920,000
Equity Securities	5,581,000	6,079,000
Estimate of Fair Value Measurement [Member] \| US Treasury Securities [Member]
Assets:
Available for sale debt securities	48,951,000
Estimate of Fair Value Measurement [Member] \| Residential Mortgage Backed Securities [Member]
Assets:
Available for sale debt securities	4,203,931,000	4,169,363,000
Estimate of Fair Value Measurement [Member] \| States and Political Subdivisions Debt Securities [Member]
Assets:
Available for sale debt securities	41,478,000	44,557,000
Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value Measurement [Member]
Assets:
Equity Securities	5,581,000	6,079,000
Marketable Securities	4,299,941,000	4,219,999,000
Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value Measurement [Member] \| US Treasury Securities [Member]
Assets:
Available for sale debt securities	48,951,000
Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value Measurement [Member] \| Residential Mortgage Backed Securities [Member]
Assets:
Available for sale debt securities	4,203,931,000	4,169,363,000
Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value Measurement [Member] \| States and Political Subdivisions Debt Securities [Member]
Assets:
Available for sale debt securities	41,478,000	44,557,000
Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member]
Assets:
Equity Securities	5,581,000	6,079,000
Marketable Securities	5,581,000	6,079,000
Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member]
Assets:
Marketable Securities	4,294,360,000	4,213,920,000
Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| US Treasury Securities [Member]
Assets:
Available for sale debt securities	48,951,000
Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| Residential Mortgage Backed Securities [Member]
Assets:
Available for sale debt securities	4,203,931,000	4,169,363,000
Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| States and Political Subdivisions Debt Securities [Member]
Assets:
Available for sale debt securities	$ 41,478,000	$ 44,557,000

Estimate of Fair Value Measurement [Member] | US Treasury Securities [Member]

Estimate of Fair Value Measurement [Member] | Residential Mortgage Backed Securities [Member]

Estimate of Fair Value Measurement [Member] | States and Political Subdivisions Debt Securities [Member]

Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]

Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | US Treasury Securities [Member]

Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Residential Mortgage Backed Securities [Member]

Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | States and Political Subdivisions Debt Securities [Member]

Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]

Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]

Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]

Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Residential Mortgage Backed Securities [Member]

Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | States and Political Subdivisions Debt Securities [Member]