Shareholders' Equity - Change in AOCI Components Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,286,832	$ 1,268,200	$ 1,231,868
Accounting Standards Update Extensible List	us-gaap:AccountingStandardsUpdate201613Member
Other Comprehensive Income (Loss)	$ 38,934	19,931	(8,851)
Balance at End of Period	1,374,507	1,286,832	1,268,200
Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	3,632
Balance at End of Period		3,632
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	8,359	(10,447)	(1,915)
Other Comprehensive Income (Loss) Before Reclassifications	43,163	$ 22,168	(8,120)
Accounting Standards Update Extensible List		us-gaap:AccountingStandardsUpdate201904Member
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(27)	$ (103)
Other Comprehensive Income (Loss)	43,136	18,806	(8,120)
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE			(412)
Balance at End of Period	51,495	8,359	(10,447)
Unrealized Gains and Losses on Net Investment Securities | Available-for-sale Securities | Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period		(3,259)
Balance at End of Period			(3,259)
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	(715)	$ (4,586)	(5,085)
Accounting Standards Update Extensible List		us-gaap:AccountingStandardsUpdate201904Member
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	292	$ 612	1,595
Other Comprehensive Income (Loss)	292	3,871	1,595
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE			(1,096)
Balance at End of Period	(423)	(715)	(4,586)
Unrealized Gains and Losses on Net Investment Securities | Held-to-maturity Securities | Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period		3,259
Balance at End of Period			3,259
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(38,756)	(36,010)	(27,715)
Other Comprehensive Income (Loss) Before Reclassifications	(6,017)	(3,709)	(3,284)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	1,523	963	958
Other Comprehensive Income (Loss)	(4,494)	(2,746)	(2,326)
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE			(5,969)
Balance at End of Period	(43,250)	(38,756)	(36,010)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(31,112)	(51,043)	(34,715)
Other Comprehensive Income (Loss) Before Reclassifications	37,146	18,459	(11,404)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	1,788	1,472	2,553
Other Comprehensive Income (Loss)	38,934	19,931	(8,851)
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE			(7,477)
Balance at End of Period	$ 7,822	$ (31,112)	$ (51,043)