Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Each Component of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realization of gain on sale of investment securities available for sale									$ 0	$ 0	$ (7)
Realization of gain on transfer of investment securities available for sale to trading									(8,148)	(10,288)	(11,546)
Total	$ (19,628)	$ (16,833)	$ (19,013)	$ 14,120	$ (20,585)	$ (20,761)	$ (20,017)	$ (13,440)	(41,354)	(74,803)	(77,957)
Income tax effect	(4,094)	(3,709)	(4,010)	2,957	(4,202)	(4,402)	(4,239)	(2,764)	(8,856)	(15,607)	(14,165)
Other operating expenses									20,123	17,981	16,739
Net of income tax	$ (15,537)	$ (13,164)	$ (15,035)	$ 11,163	$ (16,384)	$ (16,360)	$ (15,785)	$ (10,677)	(32,573)	(59,206)	(63,792)
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 on sale of investment securities available for sale									0	0	(7)
Realization of gain on transfer of investment securities available for sale to trading									0	0	(417)
Total									0	0	(424)
Income tax effect									0	0	89
Net of income tax									0	0	(335)
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									(25)	(17)	(21)
Other operating expenses									118	79	100
Net of income tax									$ 93	$ 62	$ 79