Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 94,301	$ 45,838
Investment securities AFS
Net unrealized gains (losses)	(23,979)	26,419
Amortization of net unrealized (gains) losses on AFS securities transferred to HTM securities	518	556
Reclassification adjustment for net losses (gains) realized in net income	39	(6,118)
Income tax (expense) benefit	5,851	(5,225)
Other comprehensive income (loss) on investment securities AFS	(17,571)	15,632
Defined benefit pension and postretirement obligations
Amortization of prior service cost	(37)	(38)
Amortization of actuarial loss (gain)	1,050	808
Income tax (expense) benefit	(253)	(193)
Other comprehensive income (loss) on pension and postretirement obligations	760	577
Total other comprehensive income (loss)	(16,811)	16,209
Comprehensive income	$ 77,490	$ 62,046