Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 88,809	$ 45,214	$ 274,117	$ 239,769
Investment securities AFS
Net unrealized gains (losses)	(19,827)	5,840	(35,829)	53,900
Amortization of net unrealized (gains) losses on AFS securities transferred to HTM securities	172	1,296	1,335	2,628
Reclassification adjustment for net losses (gains) realized in net income	0	(7)	16	(9,222)
Income tax (expense) benefit	4,975	(1,786)	8,548	(11,852)
Other comprehensive income (loss) on investment securities AFS	(14,681)	5,342	(25,930)	35,454
Defined benefit pension and postretirement obligations
Amortization of prior service cost	(37)	(36)	(111)	(111)
Amortization of actuarial loss (gain)	1,346	1,313	3,446	2,923
Income tax (expense) benefit	(330)	(703)	(836)	(1,088)
Other comprehensive income (loss) on pension and postretirement obligations	979	573	2,498	1,724
Total other comprehensive income (loss)	(13,702)	5,916	(23,431)	37,178
Comprehensive income	$ 75,107	$ 51,130	$ 250,685	$ 276,948