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Other Comprehensive (Loss) Income - Changes in Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]      
Balance at the beginning of the period $ 1,251,781 $ 1,196,065 $ 1,144,099
Current period change in other comprehensive income (loss) (2,673) (851) (2,575)
Reclassification due to the adoption of ASU No. 2018-02 0    
Balance at the end of the period 1,298,661 1,251,781 1,196,065
Unrealized Gains on Securities Available for Sale [Member]      
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]      
Balance at the beginning of the period (510) 3,951  
Current period change in other comprehensive income (loss) (2,163) (4,461)  
Reclassification due to the adoption of ASU No. 2018-02 (619) 0  
Balance at the end of the period (3,292) (510) 3,951
Post-Retirement Obligations [Member]      
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]      
Balance at the beginning of the period (3,056) (6,424)  
Current period change in other comprehensive income (loss) (889) 3,368  
Reclassification due to the adoption of ASU No. 2018-02 (901) 0  
Balance at the end of the period (4,846) (3,056) (6,424)
Unrealized gains (losses) on Derivatives (cash flow hedges) [Member]      
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]      
Balance at the beginning of the period 169 (73)  
Current period change in other comprehensive income (loss) 379 242  
Reclassification due to the adoption of ASU No. 2018-02 125 0  
Balance at the end of the period 673 169 (73)
Accumulated Other Comprehensive (Loss) Income [Member]      
Accumulated Other Comprehensive Income (Loss), Net Of Tax [Roll Forward]      
Balance at the beginning of the period (3,397) (2,546) 29
Current period change in other comprehensive income (loss) (2,673) (851)  
Reclassification due to the adoption of ASU No. 2018-02 (1,395) 0  
Balance at the end of the period $ (7,465) $ (3,397) $ (2,546)