Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net Income	$ 178,040	$ 226,339	$ 208,393
Unrealized gains (losses) on AFS investment securities:
Unrealized gain (loss) on securities	65,651	56,919	(24,326)
Reclassification adjustment for securities gains included in net income	(2,359)	(3,686)	(30)
Amortization of net unrealized losses on AFS securities transferred to HTM	3,448	6,285	2,098
Non-credit related unrealized (loss) gain on other-than-temporarily impaired debt securities	0	(680)	222
Net unrealized gains (losses) on AFS investment securities	66,740	58,838	(22,036)
Defined benefit pension plan and postretirement benefits:
Unrecognized pension and postretirement (cost) income	(2,532)	(937)	1,400
Amortization of net unrecognized pension and postretirement income	1,020	1,025	1,648
Net unrealized (losses) gains on defined benefit pension and postretirement plans	(1,512)	88	3,048
Other Comprehensive Income (Loss)	65,228	58,926	(18,988)
Total Comprehensive Income	$ 243,268	$ 285,265	$ 189,405