Shareholders' Equity Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Beginning Balance		$ (137)
Amortization of net unrealized losses on AFS securities transferred to HTM		3,448	$ 6,285	$ 2,098
Reclassification of stranded tax effects				0
Ending Balance		65,091	(137)
Unrealized Gains (Losses) on Investment Securities Not Other-Than-Temporarily Impaired
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Beginning Balance		14,864	(44,654)	(18,509)
Other comprehensive loss before reclassifications		65,651	56,919	(24,326)
Amounts reclassified from accumulated other comprehensive income (loss)		(2,359)	(3,686)	(30)
Amortization of net unrealized losses on AFS securities transferred to HTM		3,448	6,285	2,098
Reclassification of stranded tax effects				(3,887)
Ending Balance		81,604	14,864	(44,654)
Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Beginning Balance		0	680	458
Other comprehensive loss before reclassifications		0	(680)	222
Amounts reclassified from accumulated other comprehensive income (loss)		0	0	0
Amortization of net unrealized losses on AFS securities transferred to HTM		0	0	0
Reclassification of stranded tax effects				0
Ending Balance		0	0	680
Unrecognized Pension and Postretirement Plan Income (Cost)
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Beginning Balance		(15,001)	(15,089)	(14,923)
Other comprehensive loss before reclassifications		(2,532)	(937)	1,400
Amounts reclassified from accumulated other comprehensive income (loss)		1,020	1,025	1,648
Amortization of net unrealized losses on AFS securities transferred to HTM		0	0	0
Reclassification of stranded tax effects				(3,214)
Ending Balance		(16,513)	(15,001)	(15,089)
Accumulated Other Comprehensive (Loss) Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Beginning Balance		(137)	(59,063)	(32,974)
Other comprehensive loss before reclassifications		63,119	55,302	(22,704)
Amounts reclassified from accumulated other comprehensive income (loss)		(1,339)	(2,661)	1,618
Amortization of net unrealized losses on AFS securities transferred to HTM		3,448	6,285	2,098
Reclassification of stranded tax effects	[1]			(7,101)
Ending Balance		$ 65,091	$ (137)	$ (59,063)

[1] Result of adoption of ASU 2018-02. See Note 1 to Consolidated Financial Statements for further details.