Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 350,994	$ 306,771	$ 326,790
Investment securities AFS
Net unrealized gains (losses)	(63,714)	55,628	111,592
Amortization of net unrealized losses on AFS securities transferred to HTM securities	1,551	3,359	895
Reclassification adjustment for net losses (gains) realized in net income	16	(9,222)	(5,957)
Income tax (expense) benefit	15,557	(12,429)	(26,898)
Other comprehensive income (loss) on investment securities AFS	(46,591)	37,336	79,631
Defined benefit pension and postretirement obligations
Amortization of prior service cost	(148)	(148)	(148)
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	1,494	0	0
Net actuarial gain	25,519	7,780	16,296
Amortization of actuarial loss	4,594	3,897	476
Income tax (expense)	(7,803)	(3,064)	(4,465)
Other comprehensive income on pension and postretirement obligations	23,656	8,465	12,158
Other Comprehensive Income (Loss), Net of Tax	(22,935)	45,801	91,789
Comprehensive income	$ 328,059	$ 352,572	$ 418,579