Other Comprehensive Income (Loss) - Details about Accumulated Other Comprehensive Income (Loss) Components (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net (loss) gain on securities transactions	$ 20,981	$ 25,917	$ 25,207	$ 23,137	$ 21,595	$ 22,116	$ 21,625	$ 21,180	$ 95,242	$ 86,516	$ 87,514
Tax expense (benefit)									(42,620)	(27,045)	$ (28,962)
Net retirement plan actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax									(1,508)	(1,338)
Net retirement plan prior service credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax									(15)	(76)
Employee Benefit Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax									(1,523)	(1,414)
Tax benefit									609	565
Net of tax									(914)	(849)
Amount Reclassified from Accumulated Other Comprehensive Income | Unrealized gains and losses on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net (loss) gain on securities transactions									(407)	926
Tax expense (benefit)									163	(370)
Net of tax									$ (244)	$ 556