Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,779,309	$ 1,673,944	$ 1,741,117	$ 1,660,702
Other comprehensive income (loss) before reclassification			(19,236)	28,417
Amounts reclassified from accumulated other comprehensive income (loss)			1,207	867
Other comprehensive income (loss), net of tax amount	(8,439)	(4,881)	(18,029)	29,284
Balance	1,768,947	1,710,041	1,768,947	1,710,041
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			17,331	(8,017)
Other comprehensive income (loss) before reclassification			(19,236)	28,417
Other comprehensive income (loss), net of tax amount			(19,236)	28,417
Balance	(1,905)	20,400	(1,905)	20,400
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(18,382)	(15,583)
Amounts reclassified from accumulated other comprehensive income (loss)			1,207	867
Other comprehensive income (loss), net of tax amount			1,207	867
Balance	(17,175)	(14,716)	(17,175)	(14,716)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(10,641)	10,565	(1,051)	(23,600)
Balance	$ (19,080)	$ 5,684	$ (19,080)	$ 5,684