Shareholders' Equity - Changes in Balances of Component of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 1,660,702	$ 1,591,453
Other comprehensive income (loss), before reclassifications, net of tax amount	31,026	14,631
Amounts reclassified from accumulated other comprehensive income (loss)	272	133
Other comprehensive income (loss), net of tax amount	31,298	14,764
Balance	1,652,399	1,587,028
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	31,026	14,678
Other comprehensive income (loss), net of tax amount	31,026	14,678
Balance	23,009	(29,146)
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)	272	261
Other comprehensive income (loss), net of tax amount	272	261
Balance	(15,311)	(12,063)
Cash Flow Hedge Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance		469
Other comprehensive income (loss), before reclassifications, net of tax amount		(47)
Amounts reclassified from accumulated other comprehensive income (loss)		(128)
Other comprehensive income (loss), net of tax amount		(175)
Balance		294
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(23,600)	(55,679)
Balance	$ 7,698	$ (40,915)