Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Net unrealized gains (losses) on available-for-sale securities, beginning balance	$ 3,250	$ (17,826)	$ (10,300)
Net unrealized losses on cash flow hedging derivative instruments, beginning balance	(6,048)	(4,437)	(5,926)
Net unrecognized losses on postretirement plans, beginning balance	(3,470)	(2,157)	(3,988)
Accumulated other comprehensive (income) loss, beginning balance	(6,268)	(24,420)	(20,214)
Net Unrealized Gains (Losses) on AFS Securities
Other comprehensive income (loss) before reclassifications	26,060	20,993	(8,514)
Less: Amounts reclassified from AOCI	0	(83)	(521)
Other comprehensive income (loss)	26,060	21,076	(7,993)
Net Unrealized Losses on Cash Flow Hedges
Other comprehensive income (loss) before reclassifications	954	(2,334)	837
Less: Amounts reclassified from AOCI	(468)	(723)	(652)
Other comprehensive income (loss)	1,422	(1,611)	1,489
Defined Benefit Postretirement Plans
Other comprehensive income (loss) before reclassifications	(1,005)	(1,506)	1,368
Less: Amounts reclassified from AOCI	(531)	(193)	(463)
Other comprehensive income (loss)	(474)	(1,313)	1,831
Accumulated other comprehensive income (loss)
Other comprehensive income (loss) before reclassifications	26,009	17,153	(6,309)
Less: Amounts reclassified from AOCI	(999)	(999)	(1,636)
Other comprehensive income (loss)	27,008	18,152	(4,673)
Net unrealized gains (losses) on available-for-sale securities, ending balance	29,310	3,250	(17,826)
Net unrealized losses on cash flow hedging derivative instruments, ending balance	(4,626)	(6,048)	(4,437)
Net unrecognized losses on postretirement plans, ending balance	(3,944)	(3,470)	(2,157)
Accumulated other comprehensive (income) loss, ending balance	$ 20,740	$ (6,268)	(24,420)
Accounting Standards Update 2016-01, 2017-12 [Member]
Net Unrealized Gains (Losses) on AFS Securities
Less: Amounts reclassified from AOCI			467
Net Unrealized Losses on Cash Flow Hedges
Less: Amounts reclassified from AOCI			0
Defined Benefit Postretirement Plans
Less: Amounts reclassified from AOCI			0
Accumulated other comprehensive income (loss)
Less: Amounts reclassified from AOCI			$ 467