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	$ 306,771	$ 326,790	$ 333,562
Investment securities AFS
Net unrealized gains (losses)	55,628	111,592	(39,891)
Amortization of net unrealized (gains) losses on AFS securities transferred to HTM securities	3,359	895	(572)
Reclassification adjustment for net losses (gains) realized in net income	(9,222)	(5,957)	1,985
Reclassification from OCI due to change in accounting principle	0	0	(84)
Reclassification of certain tax effects from OCI	0	0	(8,419)
Income tax (expense) benefit	(12,429)	(26,898)	9,791
Other comprehensive income (loss) on investment securities AFS	37,336	79,631	(37,189)
Defined benefit pension and postretirement obligations
Amortization of prior service cost	(148)	(148)	(148)
Net actuarial (loss) gain	7,780	16,296	(28,612)
Amortization of actuarial loss (gain)	3,897	476	2,203
Reclassification of certain tax effects from OCI	0	0	(5,235)
Income tax (expense) benefit	(3,064)	(4,465)	6,767
Other comprehensive income (loss) on pension and postretirement obligations	8,465	12,158	(25,025)
Other Comprehensive Income (Loss), Net of Tax	45,801	91,789	(62,214)
Comprehensive income	$ 352,572	$ 418,579	$ 271,348