Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,652,399	$ 1,587,028	$ 1,660,702	$ 1,591,453
Other comprehensive income (loss), before reclassifications, net of tax amount			33,595	30,556
Amounts reclassified from accumulated other comprehensive income (loss)			570	307
Other comprehensive income (loss), net of tax amount	2,867	16,099	34,165	30,863
Balance	1,673,944	1,618,550	1,673,944	1,618,550
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			33,595	30,679
Other comprehensive income (loss), net of tax amount			33,595	30,679
Balance	25,578	(13,145)	25,578	(13,145)
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)			570	541
Other comprehensive income (loss), net of tax amount			570	541
Balance	(15,013)	(11,783)	(15,013)	(11,783)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance				469
Other comprehensive income (loss), before reclassifications, net of tax amount				(123)
Amounts reclassified from accumulated other comprehensive income (loss)				(234)
Other comprehensive income (loss), net of tax amount				(357)
Balance		112		112
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	7,698	(40,915)	(23,600)	(55,679)
Balance	$ 10,565	$ (24,816)	$ 10,565	$ (24,816)