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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

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