Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,673,944	$ 1,618,550	$ 1,660,702	$ 1,591,453
Other comprehensive income (loss), before reclassifications, net of tax amount			28,417	34,323
Amounts reclassified from accumulated other comprehensive income (loss)			867	498
Other comprehensive income (loss), net of tax amount	(4,881)	3,958	29,284	34,821
Balance	1,710,041	1,645,362	1,710,041	1,645,362
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(8,017)	(43,824)
Other comprehensive income (loss), before reclassifications, net of tax amount			28,417	34,432
Other comprehensive income (loss), net of tax amount			28,417	34,432
Balance	20,400	(9,392)	20,400	(9,392)
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(15,583)	(12,324)
Amounts reclassified from accumulated other comprehensive income (loss)			867	807
Other comprehensive income (loss), net of tax amount			867	807
Balance	(14,716)	(11,517)	(14,716)	(11,517)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance				469
Other comprehensive income (loss), before reclassifications, net of tax amount				(109)
Amounts reclassified from accumulated other comprehensive income (loss)				(309)
Other comprehensive income (loss), net of tax amount				(418)
Balance		51		51
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	10,565	(24,816)	(23,600)	(55,679)
Balance	$ 5,684	$ (20,858)	$ 5,684	$ (20,858)