Accumulated Other Comprehensive Income (Loss) - Change in AOCI Components Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,583,531	$ 1,360,221	$ 1,374,507	$ 1,352,082	$ 1,327,929	$ 1,286,832	$ 1,374,507	$ 1,286,832
Total Other Comprehensive Income (Loss)	(7,100)	319	(49,609)	(4,526)	8,104	41,933	(56,390)	45,511
Balance at End of Period	1,597,109	1,583,531	1,360,221	1,361,739	1,352,082	1,327,929	1,597,109	1,361,739
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	950		51,495	57,511		8,359	51,495	8,359
Other Comprehensive Income (Loss) Before Reclassifications	(7,544)			(4,976)			(55,406)	44,232
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(61)			0			(2,744)	(56)
Total Other Comprehensive Income (Loss)	(7,605)			(4,976)			(58,150)	44,176
Balance at End of Period	(6,655)	950		52,535	57,511		(6,655)	52,535
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	(51)		(423)	(578)		(715)	(423)	(715)
Other Comprehensive Income (Loss) Before Reclassifications	0			0			0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	64			76			436	213
Total Other Comprehensive Income (Loss)	64			76			436	213
Balance at End of Period	13	(51)		(502)	(578)		13	(502)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(42,367)		(43,250)	(38,008)		(38,756)	(43,250)	(38,756)
Other Comprehensive Income (Loss) Before Reclassifications	0			0			0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	441			374			1,324	1,122
Total Other Comprehensive Income (Loss)	441			374			1,324	1,122
Balance at End of Period	(41,926)	(42,367)		(37,634)	(38,008)		(41,926)	(37,634)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(41,468)	(41,787)	7,822	18,925	10,821	(31,112)	7,822	(31,112)
Other Comprehensive Income (Loss) Before Reclassifications	(7,544)			(4,976)			(55,406)	44,232
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	444			450			(984)	1,279
Total Other Comprehensive Income (Loss)	(7,100)	319	(49,609)	(4,526)	8,104	41,933	(56,390)	45,511
Balance at End of Period	$ (48,568)	$ (41,468)	$ (41,787)	$ 14,399	$ 18,925	$ 10,821	$ (48,568)	$ 14,399