Fair Value Measurements - Fair Value By Level (Details) - USD ($)	9 Months Ended
Sep. 30, 2026	Dec. 31, 2025
Assets:
Estimated fair value, Available-for-sale securities	$ 5,149,358,000	$ 4,966,268,000
Equity Securities	5,326,000	5,573,000
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:
Estimated fair value, Available-for-sale securities	5,149,358,000	4,966,268,000
Estimate of Fair Value Measurement \| Residential mortgage-backed securities
Assets:
Estimated fair value, Available-for-sale securities	5,031,447,000	4,830,588,000
Estimate of Fair Value Measurement \| States and political subdivisions
Assets:
Estimated fair value, Available-for-sale securities	117,911,000	135,680,000
Recurring \| Estimate of Fair Value Measurement
Assets:
Equity Securities	5,326,000	5,573,000
Marketable Securities	5,154,684,000	4,971,841,000
Recurring \| Estimate of Fair Value Measurement \| Residential mortgage-backed securities
Assets:
Estimated fair value, Available-for-sale securities	5,031,447,000	4,830,588,000
Recurring \| Estimate of Fair Value Measurement \| States and political subdivisions
Assets:
Estimated fair value, Available-for-sale securities	117,911,000	135,680,000
Recurring \| Estimate of Fair Value Measurement \| Fair Value, Inputs, Level 1
Assets:
Equity Securities	5,326,000	5,573,000
Marketable Securities	5,326,000	5,573,000
Recurring \| Estimate of Fair Value Measurement \| Fair Value, Inputs, Level 2
Assets:
Marketable Securities	5,149,358,000	4,966,268,000
Recurring \| Estimate of Fair Value Measurement \| Fair Value, Inputs, Level 2 \| Residential mortgage-backed securities
Assets:
Estimated fair value, Available-for-sale securities	5,031,447,000	4,830,588,000
Recurring \| Estimate of Fair Value Measurement \| Fair Value, Inputs, Level 2 \| States and political subdivisions
Assets:
Estimated fair value, Available-for-sale securities	$ 117,911,000	$ 135,680,000

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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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- Definition

Amount of investment in marketable security.

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