Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 54,440	$ 41,035	$ 108,808	$ 116,514
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	(5,781)	3,116	26,585	32,293
Change in net unrealized holding loss on securities transferred to held to maturity	603	637	1,832	2,139
Reclassification adjustments for changes realized in net income:
Net change in prior service costs	28	46	84	140
Recognized net loss due to lump sum settlement	30	34	60	105
Change in net actuarial loss	239	186	723	562
Derivatives:
Change in the accumulated gain (loss) on effective cash flow hedge derivatives	0	14	0	(109)
Reclassification adjustment for (gain) loss realized in net income	0	(75)	0	(309)
Other comprehensive income (loss), net of tax	(4,881)	3,958	29,284	34,821
Comprehensive income	$ 49,559	$ 44,993	$ 138,092	$ 151,335