Accumulated Other Comprehensive Income - Change in AOCI Components Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,667,774	$ 1,414,242
Other Comprehensive Income	24,992	13,107
Balance at End of Period	1,704,935	1,435,977
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	(189,230)	(224,407)
Other Comprehensive Income Before Reclassifications	20,575	8,380
Other Comprehensive Income	20,575	8,380
Balance at End of Period	(168,655)	(216,027)
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	(130,763)	(149,021)
Amounts Reclassified from Accumulated Other Comprehensive Income	4,185	4,558
Other Comprehensive Income	4,185	4,558
Balance at End of Period	(126,578)	(144,463)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(23,396)	(23,260)
Amounts Reclassified from Accumulated Other Comprehensive Income	232	169
Other Comprehensive Income	232	169
Balance at End of Period	(23,164)	(23,091)
Accum. Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(343,389)	(396,688)
Other Comprehensive Income Before Reclassifications	20,575	8,380
Amounts Reclassified from Accumulated Other Comprehensive Income	4,417	4,727
Other Comprehensive Income	24,992	13,107
Balance at End of Period	$ (318,397)	$ (383,581)