Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 91,007	$ 148,718	$ 185,307	$ 194,555
Investment securities AFS
Net unrealized gains (losses)	7,978	21,641	(16,002)	48,060
Amortization of net unrealized (gains) losses on AFS securities transferred to HTM securities	645	776	1,163	1,332
Reclassification adjustment for net losses (gains) realized in net income	(24)	(3,096)	16	(9,214)
Income tax (expense) benefit	(2,277)	(4,841)	3,574	(10,066)
Other comprehensive income (loss) on investment securities AFS	6,322	14,481	(11,249)	30,112
Defined benefit pension and postretirement obligations
Amortization of prior service cost	(37)	(38)	(74)	(75)
Amortization of actuarial loss (gain)	1,050	803	2,100	1,610
Income tax (expense) benefit	(253)	(192)	(506)	(385)
Other comprehensive income (loss) on pension and postretirement obligations	760	573	1,519	1,150
Total other comprehensive income (loss)	7,082	15,054	(9,729)	31,263
Comprehensive income	$ 98,088	$ 163,772	$ 175,578	$ 225,818