Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income		$ 326,790	$ 333,562	$ 229,264
Investment securities available for sale
Net unrealized gains (losses)		111,592	(39,891)	(12,302)
Net unrealized gain (loss) on available for sale securities transferred to held to maturity securities		0	0	(14,738)
Amortization of net unrealized (gains) losses on available for sale securities transferred to held to maturity securities		895	(572)	(2,665)
Reclassification adjustment for net losses (gains) realized in net income(a)	[1]	(5,957)	1,985	0
Reclassification from OCI due to change in accounting principle		0	(84)	0
Reclassification of certain tax effects from OCI		0	(8,419)	0
Income tax (expense) benefit		(26,898)	9,791	11,331
Other comprehensive income (loss) on investment securities available for sale		79,631	(37,189)	(18,374)
Defined benefit pension and postretirement obligations
Amortization of prior service cost		(148)	(148)	(148)
Net actuarial (loss) gain		16,296	(28,612)	14,273
Amortization of actuarial loss (gain)		476	2,203	2,282
Reclassification of certain tax effects from OCI		0	(5,235)	0
Income tax (expense) benefit		(4,465)	6,767	(6,112)
Other comprehensive income (loss) on pension and postretirement obligations		12,158	(25,025)	10,295
Total other comprehensive income (loss)		91,789	(62,214)	(8,079)
Comprehensive income		$ 418,579	$ 271,348	$ 221,185

[1] (a) Includes only available for sale securities.