Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,759,705	$ 1,652,399	$ 1,741,117	$ 1,660,702
Other comprehensive income (loss) before reclassification			(10,306)	33,595
Amounts reclassified from accumulated other comprehensive income (loss)			716	570
Other comprehensive income (loss), net of tax amount	5,865	2,867	(9,590)	34,165
Balance	1,779,309	1,673,944	1,779,309	1,673,944
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			(10,306)	33,595
Other comprehensive income (loss), net of tax amount			(10,306)	33,595
Balance	7,025	25,578	7,025	25,578
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)			716	570
Other comprehensive income (loss), net of tax amount			716	570
Balance	(17,666)	(15,013)	(17,666)	(15,013)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(16,506)	7,698	(1,051)	(23,600)
Balance	$ (10,641)	$ 10,565	$ (10,641)	$ 10,565