Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 45,214	$ 83,339	$ 239,769	$ 254,686
Investment securities available for sale
Net unrealized gains (losses)	5,840	33,173	53,900	123,139
Amortization of net unrealized (gains) losses on available for sale securities transferred to held to maturity securities	1,296	(8)	2,628	279
Reclassification adjustment for net losses (gains) realized in net income	(7)	(3,788)	(9,222)	(5,931)
Income tax (expense) benefit	(1,786)	(7,410)	(11,852)	(29,651)
Other comprehensive income (loss) on investment securities available for sale	5,342	21,967	35,454	87,836
Defined benefit pension and postretirement obligations
Amortization of prior service cost	(36)	(36)	(111)	(111)
Amortization of actuarial loss (gain)	1,313	229	2,923	357
Income tax (expense) benefit	(703)	(49)	(1,088)	(62)
Other comprehensive income (loss) on pension and postretirement obligations	573	144	1,724	184
Total other comprehensive income (loss)	5,916	22,111	37,178	88,020
Comprehensive income	$ 51,130	$ 105,450	$ 276,948	$ 342,706