Accumulated Other Comprehensive Income (Loss) - Change in AOCI Components Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,435,977	$ 1,414,242	$ 1,354,430	$ 1,316,995	$ 1,414,242	$ 1,316,995
Total Other Comprehensive Income (Loss)	9,220	13,107	(18,133)	29,360	22,327	11,227
Balance at End of Period	1,612,849	1,435,977	1,358,279	1,354,430	1,612,849	1,358,279
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	(216,027)	(224,407)	(216,588)	(240,783)	(224,407)	(240,783)
Other Comprehensive Income (Loss) Before Reclassifications	4,427		(23,215)		12,807	980
Total Other Comprehensive Income (Loss)	4,427		(23,215)		12,807	980
Balance at End of Period	(211,600)	(216,027)	(239,803)	(216,588)	(211,600)	(239,803)
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	(144,463)	(149,021)	(163,716)	(168,797)	(149,021)	(168,797)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	4,625		4,998		9,183	10,079
Total Other Comprehensive Income (Loss)	4,625		4,998		9,183	10,079
Balance at End of Period	(139,838)	(144,463)	(158,718)	(163,716)	(139,838)	(158,718)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(23,091)	(23,260)	(24,994)	(25,078)	(23,260)	(25,078)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	168		84		337	168
Total Other Comprehensive Income (Loss)	168		84		337	168
Balance at End of Period	(22,923)	(23,091)	(24,910)	(24,994)	(22,923)	(24,910)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(383,581)	(396,688)	(405,298)	(434,658)	(396,688)	(434,658)
Other Comprehensive Income (Loss) Before Reclassifications	4,427		(23,215)		12,807	980
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	4,793		5,082		9,520	10,247
Total Other Comprehensive Income (Loss)	9,220	13,107	(18,133)	29,360	22,327	11,227
Balance at End of Period	$ (374,361)	$ (383,581)	$ (423,431)	$ (405,298)	$ (374,361)	$ (423,431)