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Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Change in net unrealized loss during period $ (6,545) $ (19,379) $ 3,151  
Reclassification adjustment for net gains included in net income (718) (330) (137)  
Accumulated other comprehensive income (loss) 1,427,616 982,593 950,493 $ 908,924
Net unrealized gain (loss) on investment securities available-for-sale        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Accumulated other comprehensive income (loss) (13,200) (10,100)    
Net actuarial gain (loss) on defined benefit post-retirement benefit plans        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Accumulated other comprehensive income (loss) 1,300 2,000    
Net unrealized gain (loss) on derivatives        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Accumulated other comprehensive income (loss) 0 0    
Total other comprehensive income        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Accumulated other comprehensive income (loss) $ (11,963) $ (8,128) $ 406 $ (2,534)