Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Schedule of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 852,401	$ 764,230
Other comprehensive income (loss) before reclassifications	15,993	4,523
Amounts reclassified from accumulated other comprehensive income	137	174
Net current-period other comprehensive income (loss)	16,130	4,697
Ending balance	862,198	788,139
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	15,993	4,523
Amounts reclassified from accumulated other comprehensive income	0	0
Net current-period other comprehensive income (loss)	15,993	4,523
Ending balance	23,497	(4,971)
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	137	174
Net current-period other comprehensive income (loss)	137	174
Ending balance	(2,244)	(2,293)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	5,123	(11,961)
Ending balance	$ 21,253	$ (7,264)