Accumulated Other Comprehensive Income (Loss) - Change in AOCI Components Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,316,995	$ 1,611,611
Total Other Comprehensive Income (Loss)	29,360	(179,771)
Balance at End of Period	1,354,430	1,448,885
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	(240,783)	(32,940)
Other Comprehensive Income (Loss) Before Reclassifications	24,195	(180,173)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0
Total Other Comprehensive Income (Loss)	24,195	(180,173)
Balance at End of Period	(216,588)	(213,113)
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	(168,797)	54
Other Comprehensive Income (Loss) Before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	5,081	49
Total Other Comprehensive Income (Loss)	5,081	49
Balance at End of Period	(163,716)	103
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(25,078)	(33,496)
Other Comprehensive Income (Loss) Before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	84	353
Total Other Comprehensive Income (Loss)	84	353
Balance at End of Period	(24,994)	(33,143)
Accum.Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(434,658)	(66,382)
Other Comprehensive Income (Loss) Before Reclassifications	24,195	(180,173)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	5,165	402
Total Other Comprehensive Income (Loss)	29,360	(179,771)
Balance at End of Period	$ (405,298)	$ (246,153)