Accumulated Other Comprehensive Income (Loss) - Change in AOCI Components Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,354,430	$ 1,316,995	$ 1,448,885	$ 1,611,611	$ 1,316,995	$ 1,611,611
Total Other Comprehensive Income (Loss)	(18,133)	29,360	(122,295)	(179,771)	11,227	(302,066)
Balance at End of Period	1,358,279	1,354,430	1,348,746	1,448,885	1,358,279	1,348,746
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	(216,588)	(240,783)	(213,113)	(32,940)	(240,783)	(32,940)
Other Comprehensive Income (Loss) Before Reclassifications	(23,215)		(122,645)		980	(302,818)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0		0		0	0
Total Other Comprehensive Income (Loss)	(23,215)		(122,645)		980	(302,818)
Balance at End of Period	(239,803)	(216,588)	(335,758)	(213,113)	(239,803)	(335,758)
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	(163,716)	(168,797)	103	54	(168,797)	54
Other Comprehensive Income (Loss) Before Reclassifications	0		0		0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	4,998		(2)		10,079	47
Total Other Comprehensive Income (Loss)	4,998		(2)		10,079	47
Balance at End of Period	(158,718)	(163,716)	101	103	(158,718)	101
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(24,994)	(25,078)	(33,143)	(33,496)	(25,078)	(33,496)
Other Comprehensive Income (Loss) Before Reclassifications	0		0		0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	84		352		168	705
Total Other Comprehensive Income (Loss)	84		352		168	705
Balance at End of Period	(24,910)	(24,994)	(32,791)	(33,143)	(24,910)	(32,791)
Accum.Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(405,298)	(434,658)	(246,153)	(66,382)	(434,658)	(66,382)
Other Comprehensive Income (Loss) Before Reclassifications	(23,215)		(122,645)		980	(302,818)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	5,082		350		10,247	752
Total Other Comprehensive Income (Loss)	(18,133)	29,360	(122,295)	(179,771)	11,227	(302,066)
Balance at End of Period	$ (423,431)	$ (405,298)	$ (368,448)	$ (246,153)	$ (423,431)	$ (368,448)

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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