Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 148,718	$ 84,661	$ 194,555	$ 171,347
Investment securities available for sale
Net unrealized gains (losses)	21,641	59,476	48,060	89,966
Amortization of net unrealized (gains) losses on available for sale securities transferred to held to maturity securities	776	218	1,332	288
Reclassification adjustment for net losses (gains) realized in net income	(3,096)	(463)	(9,214)	(2,143)
Income tax (expense) benefit	(4,841)	(14,940)	(10,066)	(22,242)
Other comprehensive income (loss) on investment securities available for sale	14,481	44,292	30,112	65,869
Defined benefit pension and postretirement obligations
Amortization of prior service cost	(38)	(38)	(75)	(75)
Amortization of actuarial loss (gain)	803	64	1,610	128
Income tax (expense) benefit	(192)	(7)	(385)	(13)
Other comprehensive income (loss) on pension and postretirement obligations	573	20	1,150	40
Total other comprehensive income (loss)	15,054	44,311	31,263	65,909
Comprehensive income	$ 163,772	$ 128,972	$ 225,818	$ 237,256