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Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]      
Balance at the beginning of the period $ 1,413,840 $ 1,358,980 $ 1,298,661
Current period change in other comprehensive income (loss) 13,834 16,157 (4,687)
Balance at the end of the period 1,619,797 1,413,840 1,358,980
Cumulative Effect, Period of Adoption, Adjustment      
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]      
Balance at the beginning of the period (8,311) 4,350 0
Balance at the end of the period   (8,311) 4,350
Unrealized Losses on Available for Sale Debt Securities      
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]      
Balance at the beginning of the period 8,746 (9,605)  
Current period change in other comprehensive income (loss) 14,944 18,351  
Balance at the end of the period 23,690 8,746 (9,605)
Post-Retirement Obligations      
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]      
Balance at the beginning of the period (5,240) (3,625)  
Current period change in other comprehensive income (loss) 4,159 (1,615)  
Balance at the end of the period (1,081) (5,240) (3,625)
Unrealized gains (losses) on Derivatives (cash flow hedges)      
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]      
Balance at the beginning of the period 315 894  
Current period change in other comprehensive income (loss) (5,269) (579)  
Balance at the end of the period (4,954) 315 894
Accumulated Other Comprehensive Loss      
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]      
Balance at the beginning of the period 3,821 (12,336) (7,465)
Current period change in other comprehensive income (loss) 13,834 16,157 (4,687)
Balance at the end of the period $ 17,655 3,821 (12,336)
Accumulated Other Comprehensive Loss | Cumulative Effect, Period of Adoption, Adjustment      
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]      
Balance at the beginning of the period   $ 0 (184)
Balance at the end of the period     $ 0