Other Comprehensive Income (Loss) - Schedule of Amounts Reclassified Out of Each Component of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net gain on securities transactions	$ 27,003	$ 24,167	$ 36,898	$ 50,834
Tax (expense) benefit	(5,540)	(4,743)	(7,449)	(10,338)
Other operating expense	(12,662)	(12,648)	(24,537)	(24,153)
Net of tax	21,431	19,392	29,380	40,432
Amount Reclassified from Accumulated Other Comprehensive (Loss) Income | Available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net gain on securities transactions	0	271	429	271
Tax (expense) benefit	0	(67)	(105)	(67)
Net of tax	0	204	324	204
Amount Reclassified from Accumulated Other Comprehensive (Loss) Income | Net retirement plan actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other operating expense	(581)	(400)	(1,183)	(821)
Amount Reclassified from Accumulated Other Comprehensive (Loss) Income | Net retirement plan prior service cost
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other operating expense	(54)	(3)	(107)	(7)
Amount Reclassified from Accumulated Other Comprehensive (Loss) Income | Employee benefit plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net gain on securities transactions	(635)	(403)	(1,290)	(828)
Tax (expense) benefit	155	99	316	203
Net of tax	$ (480)	$ (304)	$ (974)	$ (625)