Accumulated Other Comprehensive Income (Loss) (Schedule of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 852,401	$ 764,230
Other comprehensive income (loss) before reclassifications	18,128	13,510
Amounts reclassified from accumulated other comprehensive income	(5,905)	339
Net current-period other comprehensive income (loss)	12,223	13,849
Ending balance	867,891	815,169
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	18,128	13,510
Amounts reclassified from accumulated other comprehensive income	(6,180)	0
Net current-period other comprehensive income (loss)	11,948	13,510
Ending balance	19,452	4,016
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	275	339
Net current-period other comprehensive income (loss)	275	339
Ending balance	(2,106)	(2,128)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	5,123	(11,961)
Ending balance	$ 17,346	$ 1,888