Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,741,311	$ 1,741,117
Other comprehensive income (loss) before reclassification	(116,353)	15,817
Amounts reclassified from accumulated other comprehensive income (loss)	257	362
Other comprehensive income (loss), net of tax amount	(116,096)	(15,455)
Balance	1,631,382	1,759,705
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	17,774	(17,331)
Other comprehensive income (loss) before reclassification	(116,353)	15,817
Other comprehensive income (loss), net of tax amount	(116,353)	15,817
Balance	(134,127)	1,514
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(14,786)	(18,382)
Amounts reclassified from accumulated other comprehensive income (loss)	257	362
Other comprehensive income (loss), net of tax amount	257	362
Balance	(14,529)	(18,020)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(32,560)	(1,051)
Balance	$ (148,656)	$ (16,506)