Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 124,073	$ 142,684	$ 424,177	$ 353,639
Unrealized (losses) gains on securities available for sale:
Unrealized (losses) gains on securities available for sale arising during the period	(11,270)	9,781	(43,173)	182,690
Tax effect	2,593	(2,251)	9,930	(42,019)
Reclassification adjustment for realized gains on securities available for sale included in income before income taxes	(8,082)	(21,425)	(33,119)	(54,972)
Tax effect	1,858	4,928	7,617	12,644
Total change in unrealized (losses) gains on securities available for sale, net of tax	(14,901)	(8,967)	(58,745)	98,343
Unrealized losses on securities available for sale transferred to held to maturity:
Reclassification adjustment for accretion of unrealized losses on securities available for sale transferred to held to maturity	(430)	0	(1,690)	0
Tax effect	99	0	389	0
Total change in unrealized losses on securities available for sale transferred to held to maturity, net of tax	(331)	0	(1,301)	0
Change in pension obligation:
Amortization of actuarial losses	6,773	6,332	20,320	18,994
Tax effect	(1,558)	(1,457)	(4,674)	(4,370)
Total change in pension obligation, net of tax	5,215	4,875	15,646	14,624
Net current period other comprehensive (loss) income	(10,017)	(4,092)	(44,400)	112,967
Total comprehensive income	$ 114,056	$ 138,592	$ 379,777	$ 466,606