Other Comprehensive Loss - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net gain on securities transactions									$ 2,221	$ 57	$ 64
Income tax expense (benefit)	$ (6,026)	$ (8,575)	$ (4,568)	$ (6,361)	$ (15,740)	$ (11,969)	$ (10,451)	$ (8,368)	(25,530)	(46,528)	(36,980)
Net of tax	$ 35,763	$ 35,468	$ 19,243	$ 27,913	$ 19,483	$ 26,575	$ 24,383	$ 23,508	118,387	93,949	87,802
Total Reclassifications									290	179	594
Securities available for sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net gain on securities transactions									0	0	50
Income tax expense (benefit)									0	0	(20)
Net of tax									0	0	30
Post-retirement obligations - Amortization of actuarial losses [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Compensation and employee benefits									399	243	943
Income tax expense									(109)	(64)	(379)
Total Reclassifications									$ 290	$ 179	$ 564