Accumulated Other Comprehensive Income (Loss) - Change in AOCI Components Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2022	Jun. 30, 2021
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	$ 1,448,885	$ 1,611,611	$ 1,360,221	$ 1,374,507	$ 1,611,611	$ 1,374,507
Total Other Comprehensive Income (Loss)	(122,295)	(179,771)	319	(49,609)	(302,066)	(49,290)
Balance	1,348,746	1,448,885	1,583,531	1,360,221	1,348,746	1,583,531
Unrealized Gains and Losses on Net Investment Securities | Available-for-sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(213,113)	(32,940)	1,352	51,495	(32,940)	51,495
Other Comprehensive Income (Loss) Before Reclassifications	(122,645)		2,281		(302,818)	(47,862)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0		(2,683)		0	(2,683)
Total Other Comprehensive Income (Loss)	(122,645)		(402)		(302,818)	(50,545)
Balance	(335,758)	(213,113)	950	1,352	(335,758)	950
Unrealized Gains and Losses on Net Investment Securities | Held-to-maturity Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	103	54	(330)	(423)	54	(423)
Other Comprehensive Income (Loss) Before Reclassifications	0		0		0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(2)		279		47	372
Total Other Comprehensive Income (Loss)	(2)		279		47	372
Balance	101	103	(51)	(330)	101	(51)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(33,143)	(33,496)	(42,809)	(43,250)	(33,496)	(43,250)
Other Comprehensive Income (Loss) Before Reclassifications	0		0		0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	352		442		705	883
Total Other Comprehensive Income (Loss)	352		442		705	883
Balance	(32,791)	(33,143)	(42,367)	(42,809)	(32,791)	(42,367)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(246,153)	(66,382)	(41,787)	7,822	(66,382)	7,822
Other Comprehensive Income (Loss) Before Reclassifications	(122,645)		2,281		(302,818)	(47,862)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	350		(1,962)		752	(1,428)
Total Other Comprehensive Income (Loss)	(122,295)	(179,771)	319	(49,609)	(302,066)	(49,290)
Balance	$ (368,448)	$ (246,153)	$ (41,468)	$ (41,787)	$ (368,448)	$ (41,468)