Other Comprehensive Income (Loss) - Reclassified Out of Each Component of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net gain on securities transactions	$ 26,667	$ 26,229
Tax expense	(5,595)	(5,761)
Other operating expense	(10,923)	(10,608)
Net of tax	(21,040)	(20,436)
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	124
Tax expense	0	(30)
Net of tax	0	94
Amount Reclassified from Accumulated Other Comprehensive (Loss) Income | Net retirement plan actuarial (gain) loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other operating expense	(421)	(417)
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	(4)	(4)
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	(425)	(421)
Tax expense	104	103
Net of tax	$ 321	$ 318