Accumulated Other Comprehensive Income (Loss) - Change in AOCI Components Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,374,507	$ 1,286,832
Total Other Comprehensive Income (Loss)	(49,609)	41,933
Balance at End of Period	1,360,221	1,327,929
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	51,495	8,359
Other Comprehensive Income (Loss) Before Reclassifications	(50,143)	41,567
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	(56)
Total Other Comprehensive Income (Loss)	(50,143)	41,511
Balance at End of Period	1,352	49,870
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	(423)	(715)
Other Comprehensive Income (Loss) Before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	93	48
Total Other Comprehensive Income (Loss)	93	48
Balance at End of Period	(330)	(667)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(43,250)	(38,756)
Other Comprehensive Income (Loss) Before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	441	374
Total Other Comprehensive Income (Loss)	441	374
Balance at End of Period	(42,809)	(38,382)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	7,822	(31,112)
Other Comprehensive Income (Loss) Before Reclassifications	(50,143)	41,567
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	534	366
Total Other Comprehensive Income (Loss)	(49,609)	41,933
Balance at End of Period	$ (41,787)	$ 10,821