Accumulated Other Comprehensive Income Components of Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		$ 2,308,798	$ 2,245,785	$ 2,247,573	$ 2,229,857
Amortization of net unrealized losses on AFS securities transferred to HTM				5,425
Ending Balance		2,324,016	2,283,014	2,324,016	2,283,014
Unrealized Gains (Losses) on Investment Securities Not Other-Than-Temporarily Impaired
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		2,368	(56,690)	(44,654)	(18,509)
Other comprehensive income before reclassifications		18,029	(12,531)	63,244	(46,806)
Amounts reclassified from accumulated other comprehensive income (loss)		(3,498)	(11)	(3,686)	(30)
Amortization of net unrealized losses on AFS securities transferred to HTM				5,425	791
Reclassification of stranded tax effects					(3,887)
Ending Balance		20,329	(68,441)	20,329	(68,441)
Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		(2)	690	680	458
Other comprehensive income before reclassifications		0	0	(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	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		(14,523)	(17,258)	(15,089)	(14,923)
Other comprehensive income before reclassifications		0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)		277	369	843	1,248
Amortization of net unrealized losses on AFS securities transferred to HTM				0	0
Reclassification of stranded tax effects					(3,214)
Ending Balance		(14,246)	(16,889)	(14,246)	(16,889)
Total
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		(12,157)	(73,258)	(59,063)	(32,974)
Other comprehensive income before reclassifications		18,029	(12,531)	62,562	(46,574)
Amounts reclassified from accumulated other comprehensive income (loss)		(3,221)	358	(2,843)	1,218
Amortization of net unrealized losses on AFS securities transferred to HTM				5,425	791
Reclassification of stranded tax effects	[1]				(7,101)
Ending Balance		6,081	(84,640)	$ 6,081	$ (84,640)
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)		3,430	791
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)		3,430	791
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	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	$ 0

[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.