Accumulated Other Comprehensive Income (Loss) - Schedule of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 852,401	$ 764,230	$ 692,979
Other comprehensive income (loss) before reclassifications	14,879	16,545	(7,831)
Amounts reclassified from accumulated other comprehensive income	(5,652)	539	16
Net current-period other comprehensive income (loss)	9,227	17,084	(7,815)
Ending balance	893,421	852,401	764,230
Unrealized Gain (Loss) on Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	7,504	(9,494)	(1,694)
Other comprehensive income (loss) before reclassifications	14,425	17,073	(7,800)
Amounts reclassified from accumulated other comprehensive income	(6,180)	(75)	0
Net current-period other comprehensive income (loss)	8,245	16,998	(7,800)
Ending balance	15,749	7,504	(9,494)
Postretirement Plans Asset (Liability)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(2,381)	(2,467)	(2,452)
Other comprehensive income (loss) before reclassifications	454	(528)	(31)
Amounts reclassified from accumulated other comprehensive income	528	614	16
Net current-period other comprehensive income (loss)	982	86	(15)
Ending balance	(1,399)	(2,381)	(2,467)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	5,123	(11,961)	(4,146)
Ending balance	$ 14,350	$ 5,123	$ (11,961)