Accumulated Other Comprehensive Income Components of Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		$ 2,301,019	$ 2,235,493	$ 2,247,573	$ 2,229,857
Amortization of net unrealized losses on AFS securities transferred to HTM				1,995
Ending Balance		2,308,798	2,245,785	2,308,798	2,245,785
Unrealized Gains (Losses) on Investment Securities Not Other-Than-Temporarily Impaired
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		(23,433)	(50,056)	(44,654)	(18,509)
Other comprehensive income before reclassifications		24,917	(6,631)	45,215	(34,275)
Amounts reclassified from accumulated other comprehensive income (loss)		(137)	(3)	(188)	(19)
Amortization of net unrealized losses on AFS securities transferred to HTM				1,995
Reclassification of stranded tax effects					(3,887)
Ending Balance		2,368	(56,690)	2,368	(56,690)
Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		598	682	680	458
Other comprehensive income before reclassifications		(600)	8	(682)	232
Amounts reclassified from accumulated other comprehensive income (loss)		0	0	0	0
Amortization of net unrealized losses on AFS securities transferred to HTM				0
Reclassification of stranded tax effects					0
Ending Balance		(2)	690	(2)	690
Unrecognized Pension and Postretirement Plan Income (Costs)
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		(14,798)	(17,798)	(15,089)	(14,923)
Other comprehensive income before reclassifications		0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)		275	540	566	879
Amortization of net unrealized losses on AFS securities transferred to HTM				0
Reclassification of stranded tax effects					(3,214)
Ending Balance		(14,523)	(17,258)	(14,523)	(17,258)
Total
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		(37,633)	(67,172)	(59,063)	(32,974)
Other comprehensive income before reclassifications		24,317	(6,623)	44,533	(34,043)
Amounts reclassified from accumulated other comprehensive income (loss)		138	537	378	860
Amortization of net unrealized losses on AFS securities transferred to HTM				1,995
Reclassification of stranded tax effects	[1]				(7,101)
Ending Balance		(12,157)	$ (73,258)	$ (12,157)	$ (73,258)
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)		1,021
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains (Losses) on Investment Securities Not Other-Than-Temporarily Impaired
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)		1,021
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)		0
Reclassification out of Accumulated Other Comprehensive Income | Unrecognized Pension and Postretirement Plan Income (Costs)
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)		$ 0

[1] (1) The Corporation adopted the Accounting Standards Codification ("ASC") Update 2018-02, "Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income" in the first quarter of 2018 which permitted a reclassification from accumulated other comprehensive income ("AOCI") to retained earnings of the stranded tax effects resulting from the Tax Cuts and Jobs Act of 2017, which changed the federal corporate income tax rate from 35% to 21%. As a result, $7.1 million of stranded tax effects were reclassified from AOCI to retained earnings during the first quarter of 2018.