Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 83,339	$ 85,929	$ 254,686	$ 244,578
Investment securities available for sale
Net unrealized gains (losses)	33,173	(21,345)	123,139	(82,099)
Amortization of net unrealized (gains) losses on available for sale securities transferred to held to maturity securities	(8)	(52)	279	(684)
Reclassification adjustment for net losses (gains) realized in net income	(3,788)	(30)	(5,931)	1,985
Reclassification from OCI due to change in accounting principle	0	0	0	(84)
Reclassification of certain tax effects from OCI	0	0	0	(8,419)
Income tax (expense) benefit	(7,410)	5,456	(29,651)	20,796
Other comprehensive income (loss) on investment securities available for sale	21,967	(15,971)	87,836	(68,505)
Defined benefit pension and postretirement obligations
Amortization of prior service cost	(36)	(37)	(111)	(112)
Amortization of actuarial loss (gain)	229	551	357	1,480
Reclassification of certain tax effects from OCI	0	0	0	(5,235)
Income tax (expense) benefit	(49)	(174)	(62)	(390)
Other comprehensive income (loss) on pension and postretirement obligations	144	340	184	(4,257)
Total other comprehensive income (loss)	22,111	(15,631)	88,020	(72,762)
Comprehensive income	$ 105,450	$ 70,298	$ 342,706	$ 171,816