Accumulated Other Comprehensive Income (Loss) (Schedule of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 852,401	$ 764,230
Other comprehensive income (loss) before reclassifications	18,143	15,212
Amounts reclassified from accumulated other comprehensive income	(5,787)	383
Net current-period other comprehensive income (loss)	12,356	15,595
Ending balance	880,818	834,982
Unrealized Gain (Loss) on Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	7,504	(9,494)
Other comprehensive income (loss) before reclassifications	18,143	15,212
Amounts reclassified from accumulated other comprehensive income	(6,180)	(75)
Net current-period other comprehensive income (loss)	11,963	15,137
Ending balance	19,467	5,643
Postretirement Plans Asset (Liability)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(2,381)	(2,467)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	393	458
Net current-period other comprehensive income (loss)	393	458
Ending balance	(1,988)	(2,009)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	5,123	(11,961)
Ending balance	$ 17,479	$ 3,634