Accumulated Other Comprehensive Loss - Amounts Reclassified from Each Component of Accumulated Other Comprehensive (Loss) 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 [Line Items]
Realization of gain (loss) on sale of investment securities available for sale									$ (7)	$ (101)	$ 77
Realization of gain on transfer of investment securities available for sale to trading									(11,546)	(5,321)	(4,868)
Total	$ (18,877)	$ (20,747)	$ (18,418)	$ (19,915)	$ (13,724)	$ (15,505)	$ (14,338)	$ (13,679)	(77,957)	(57,246)	(54,204)
Income tax effect	(1,746)	(4,066)	(3,723)	(4,630)	(19,924)	(4,766)	(4,905)	(4,635)	(14,165)	(34,230)	(18,168)
Net income	$ 17,136	$ 16,682	$ 14,688	$ 15,286	$ (6,200)	$ 10,739	$ 9,433	$ 9,044	63,792	23,016	36,036
Other operating expenses									16,635	15,248	13,935
Net Change in Unrealized Gains on Available-for-Sale Investment Securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realization of gain (loss) on sale of investment securities available for sale									7	101	(77)
Realization of gain on transfer of investment securities available for sale to trading									417	0	0
Total									424	101	(77)
Income tax effect									89	35	(27)
Net income									(335)	(66)	50
Net Change in Unfunded Pension Liability | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax effect									(21)	(33)	(34)
Net income									(79)	(62)	(64)
Other operating expenses									$ 100	$ 95	$ 98