Accumulated Other Comprehensive Income (Loss) - Change in AOCI Components Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,611,611	$ 1,374,507
Total Other Comprehensive Loss	(179,771)	(49,609)
Balance at End of Period	1,448,885	1,360,221
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	(32,940)	51,495
Other Comprehensive Income (Loss) Before Reclassifications	(180,173)	(50,143)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0
Total Other Comprehensive Loss	(180,173)	(50,143)
Balance at End of Period	(213,113)	1,352
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	54	(423)
Other Comprehensive Income (Loss) Before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	49	93
Total Other Comprehensive Loss	49	93
Balance at End of Period	103	(330)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(33,496)	(43,250)
Other Comprehensive Income (Loss) Before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	353	441
Total Other Comprehensive Loss	353	441
Balance at End of Period	(33,143)	(42,809)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(66,382)	7,822
Other Comprehensive Income (Loss) Before Reclassifications	(180,173)	(50,143)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	402	534
Total Other Comprehensive Loss	(179,771)	(49,609)
Balance at End of Period	$ (246,153)	$ (41,787)