Fair Value Measurements - Fair Value By Level (Details) - USD ($)	6 Months Ended
Jun. 30, 2025	Dec. 31, 2024
Assets:
Available for sale debt securities	$ 5,009,843,000	$ 4,987,916,000
Equity Securities	5,483,000	5,394,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net	0
Estimate of Fair Value Measurement
Assets:
Available for sale debt securities	5,009,843,000	4,987,916,000
Estimate of Fair Value Measurement \| Residential mortgage-backed securities
Assets:
Available for sale debt securities	4,869,078,000	4,835,176,000
Estimate of Fair Value Measurement \| States and political subdivisions
Assets:
Available for sale debt securities	140,765,000	152,740,000
Recurring \| Estimate of Fair Value Measurement
Assets:
Equity Securities	5,483,000	5,394,000
Marketable Securities	5,015,326,000	4,993,310,000
Recurring \| Estimate of Fair Value Measurement \| Residential mortgage-backed securities
Assets:
Available for sale debt securities	4,869,078,000	4,835,176,000
Recurring \| Estimate of Fair Value Measurement \| States and political subdivisions
Assets:
Available for sale debt securities	140,765,000	152,740,000
Recurring \| Estimate of Fair Value Measurement \| Fair Value, Inputs, Level 1
Assets:
Equity Securities	5,483,000	5,394,000
Marketable Securities	5,483,000	5,394,000
Recurring \| Estimate of Fair Value Measurement \| Fair Value, Inputs, Level 2
Assets:
Marketable Securities	5,009,843,000	4,987,916,000
Recurring \| Estimate of Fair Value Measurement \| Fair Value, Inputs, Level 2 \| Residential mortgage-backed securities
Assets:
Available for sale debt securities	4,869,078,000	4,835,176,000
Recurring \| Estimate of Fair Value Measurement \| Fair Value, Inputs, Level 2 \| States and political subdivisions
Assets:
Available for sale debt securities	$ 140,765,000	$ 152,740,000

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as a liability into (out of) level 3 of the fair value hierarchy.

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Amount of investment in marketable security.

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