Other Comprehensive Income (Loss) - Reclassified Out of Each Component of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net gain on securities transactions	$ 26,361	$ 25,917	$ 80,432	$ 74,261
Tax expense	(5,427)	(8,491)	(16,939)	(24,127)
Other operating expense	(11,781)	(9,858)	(34,825)	(29,738)
Net of tax	(20,902)	(17,394)	(63,397)	(50,037)
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	30	423	324	423
Tax expense	(7)	(169)	(79)	(169)
Net of tax	(23)	(254)	(245)	(254)
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	(430)	(376)	(1,289)	(1,130)
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)	(3)	(12)	(11)
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	(434)	(379)	(1,301)	(1,141)
Tax expense	106	150	319	455
Net of tax	$ (328)	$ (229)	$ (982)	$ (686)