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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