Accumulated Other Comprehensive Income (Loss) (Schedule of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 893,421	$ 852,401
Other comprehensive income (loss) before reclassifications	(18,666)	15,993
Amounts reclassified from accumulated other comprehensive income	131	137
Net current-period other comprehensive income (loss)	(18,535)	16,130
Ending balance	876,853	862,198
Unrealized Gain (Loss) on Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	15,749	7,504
Other comprehensive income (loss) before reclassifications	(18,666)	15,993
Amounts reclassified from accumulated other comprehensive income	0	0
Net current-period other comprehensive income (loss)	(18,666)	15,993
Ending balance	(2,917)	23,497
Postretirement Plans Asset (Liability)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,399)	(2,381)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	131	137
Net current-period other comprehensive income (loss)	131	137
Ending balance	(1,268)	(2,244)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	14,350	5,123
Ending balance	$ (4,185)	$ 21,253