Accumulated Other Comprehensive Income (Loss) - Change in AOCI Components Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,612,849	$ 1,358,279	$ 1,414,242	$ 1,316,995
Total Other Comprehensive Income (Loss)	39,001	(18,180)	61,328	(6,953)
Balance at End of Period	1,665,474	1,363,840	1,665,474	1,363,840
Unrealized Gains and Losses on Net Investment Securities \| Available-for-sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(211,600)	(239,803)	(224,407)	(240,783)
Other Comprehensive Income (Loss) Before Reclassifications	34,271	(26,596)	47,078	(25,616)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		3,345		3,345
Total Other Comprehensive Income (Loss)	34,271	(23,251)	47,078	(22,271)
Balance at End of Period	(177,329)	(263,054)	(177,329)	(263,054)
Unrealized Gains and Losses on Net Investment Securities \| Held-to-maturity Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(139,838)	(158,718)	(149,021)	(168,797)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	4,562	4,987	13,745	15,066
Total Other Comprehensive Income (Loss)	4,562	4,987	13,745	15,066
Balance at End of Period	(135,276)	(153,731)	(135,276)	(153,731)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(22,923)	(24,910)	(23,260)	(25,078)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	168	84	505	252
Total Other Comprehensive Income (Loss)	168	84	505	252
Balance at End of Period	(22,755)	(24,826)	(22,755)	(24,826)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(374,361)	(423,431)	(396,688)	(434,658)
Other Comprehensive Income (Loss) Before Reclassifications	34,271	(26,596)	47,078	(25,616)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	4,730	8,416	14,250	18,663
Total Other Comprehensive Income (Loss)	39,001	(18,180)	61,328	(6,953)
Balance at End of Period	$ (335,360)	$ (441,611)	$ (335,360)	$ (441,611)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount of equity (deficit) attributable to parent. Excludes temporary equity and equity attributable to noncontrolling interest.

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