Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 150,460	$ 149,584	$ 105,630
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	33,103	(14,416)	(8,641)
Reclassification adjustment for net (gains) losses realized in net income	0	0	(9)
Change in net unrealized holding loss on securities transferred to held to maturity	2,704	2,821	2,915
Pension and other postretirement benefit plans:
Change in the actuarial loss of pension and other postretirement benefit plans	(4,278)	2,806	(760)
Reclassification adjustments for changes realized in net income:
Net change in prior service costs	187	187	154
Recognized net loss due to lump sum settlements	235	122	0
Change in net actuarial loss	597	919	1,211
Recognized net loss due to defined benefit plan termination	0	0	10,907
Derivatives:
Change in the accumulated gain (loss) on effective cash flow hedge derivatives	(109)	373	122
Reclassification adjustment for (gain) loss realized in net income	(360)	(242)	174
Other comprehensive income (loss), net of tax	32,079	(7,430)	6,073
Comprehensive income	$ 182,539	$ 142,154	$ 111,703