Accumulated Other Comprehensive Income (Loss) - Change in AOCI Components Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,358,279	$ 1,354,430	$ 1,316,995	$ 1,348,746	$ 1,448,885	$ 1,611,611	$ 1,316,995	$ 1,611,611
Total Other Comprehensive Income (Loss)	(18,180)	(18,133)	29,360	(79,246)	(122,295)	(179,771)	(6,953)	(381,312)
Balance at End of Period	1,363,840	1,358,279	1,354,430	1,282,384	1,348,746	1,448,885	1,363,840	1,282,384
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	(239,803)		(240,783)	(335,758)		(32,940)	(240,783)	(32,940)
Other Comprehensive Income (Loss) Before Reclassifications	(26,596)			(81,832)			(25,616)	(384,650)
Unrealized Net Losses Related to the Transfer of Securities from Available-for-Sale to Held-to-Maturity				176,700				176,700
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	3,345			0			3,345	0
Total Other Comprehensive Income (Loss)	(23,251)			94,868			(22,271)	(207,950)
Balance at End of Period	(263,054)	(239,803)		(240,890)	(335,758)		(263,054)	(240,890)
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	(158,718)		(168,797)	101		54	(168,797)	54
Other Comprehensive Income (Loss) Before Reclassifications	0			0			0	0
Unrealized Net Losses Related to the Transfer of Securities from Available-for-Sale to Held-to-Maturity				(176,700)				(176,700)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	4,987			2,232			15,066	2,279
Total Other Comprehensive Income (Loss)	4,987			(174,468)			15,066	(174,421)
Balance at End of Period	(153,731)	(158,718)		(174,367)	101		(153,731)	(174,367)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(24,910)		(25,078)	(32,791)		(33,496)	(25,078)	(33,496)
Other Comprehensive Income (Loss) Before Reclassifications	0			0			0	0
Unrealized Net Losses Related to the Transfer of Securities from Available-for-Sale to Held-to-Maturity				0				0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	84			354			252	1,059
Total Other Comprehensive Income (Loss)	84			354			252	1,059
Balance at End of Period	(24,826)	(24,910)		(32,437)	(32,791)		(24,826)	(32,437)
Accum.Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(423,431)	(405,298)	(434,658)	(368,448)	(246,153)	(66,382)	(434,658)	(66,382)
Other Comprehensive Income (Loss) Before Reclassifications	(26,596)			(81,832)			(25,616)	(384,650)
Unrealized Net Losses Related to the Transfer of Securities from Available-for-Sale to Held-to-Maturity				0				0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	8,416			2,586			18,663	3,338
Total Other Comprehensive Income (Loss)	(18,180)	(18,133)	29,360	(79,246)	(122,295)	(179,771)	(6,953)	(381,312)
Balance at End of Period	$ (441,611)	$ (423,431)	$ (405,298)	$ (447,694)	$ (368,448)	$ (246,153)	$ (441,611)	$ (447,694)