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Other Comprehensive (Loss) Income - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Net gain on securities transactions                 $ 57 $ 64 $ 654
Income tax expense (benefit) $ (15,740) $ (11,969) $ (10,451) $ (8,368) $ (10,182) $ (9,281) $ (8,781) $ (8,736) (46,528) (36,980) (36,441)
Net of tax $ 19,483 $ 26,575 $ 24,383 $ 23,508 $ 22,567 $ 22,906 $ 21,356 $ 20,973 93,949 87,802 83,722
Total Reclassifications                 179 594 854
Total                 (2,163) (4,461) (3,792)
Securities available for sale [Member]                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Net gain on securities transactions                 0 50 654
Income tax expense (benefit)                 0 (20) (263)
Net of tax                 0 30 391
Post-retirement obligations - Amortization of actuarial losses [Member]                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Compensation and employee benefits                 243 943 774
Income tax expense                 (64) (379) (311)
Total Reclassifications                 $ 179 $ 564 $ 463