Accumulated Other Comprehensive Income (Loss) - AOCI Reclass to IS (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Provision for Income Tax	$ (19,985)		$ (9,759)		$ (39,010)	$ (17,220)
Net Income	67,533	$ 59,949	38,908	$ 34,742	127,482	73,650
Salaries and Benefits	(56,161)		(50,715)		(112,412)	(105,178)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Income	1,962		(463)		1,428	(829)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of Unrealized Gains(Losses) of Investment Securities Transferred from AFS to HTM
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Interest Income	(380)		(121)		(506)	(186)
Provision for Income Tax	101		32		134	49
Net Income	(279)		(89)		(372)	(137)
Reclassification out of Accumulated Other Comprehensive Income | Sale of Investment Securities Available-for-Sale
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Provision for Income Tax	(992)		0		(992)	(21)
Net Income	2,683		0		2,683	56
Sale of Investment Securities Available-for-Sale	3,675		0		3,675	77
Reclassification out of Accumulated Other Comprehensive Income | Prior Service Credit
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Salaries and Benefits	62		62		123	123
Reclassification out of Accumulated Other Comprehensive Income | Net Actuarial Losses
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Salaries and Benefits	(661)		(571)		(1,323)	(1,141)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of Defined Benefit Plan Items
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Provision for Income Tax	157		135		317	270
Net Income	(442)		(374)		(883)	(748)
Salaries and Benefits	$ (599)		$ (509)		$ (1,200)	$ (1,018)