Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,631,382	$ 1,759,705	$ 1,741,311	$ 1,741,117
Other comprehensive income (loss) before reclassification			(175,088)	10,306
Amounts reclassified from accumulated other comprehensive income (loss)			506	716
Other comprehensive income (loss), net of tax amount	(58,486)	5,865	(174,582)	(9,590)
Balance	1,586,696	1,779,309	1,586,696	1,779,309
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			(175,088)	(10,306)
Amounts reclassified from accumulated other comprehensive income (loss)			0	0
Other comprehensive income (loss), net of tax amount			(175,088)	(10,306)
Balance	(192,862)	7,025	(192,862)	7,025
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(14,786)	(18,382)
Other comprehensive income (loss) before reclassification			0	0
Amounts reclassified from accumulated other comprehensive income (loss)			506	716
Other comprehensive income (loss), net of tax amount			506	716
Balance	(14,280)	(17,666)	(14,280)	(17,666)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(148,656)	(16,506)	(32,560)	(1,051)
Balance	$ (207,142)	$ (10,641)	$ (207,142)	$ (10,641)