Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 149,584	$ 105,630	$ 108,411
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	(14,416)	(8,641)	(9,667)
Reclassification adjustment for net (gains) losses realized in net income	0	(9)	191
Change in net unrealized holding loss on securities transferred to held to maturity	2,821	2,915	6,070
Pension and other postretirement benefit plans:
Change in the actuarial loss of pension and other postretirement benefit plans	2,806	(760)	(148)
Reclassification adjustments for changes realized in net income:
Net change in prior service costs	187	154	154
Recognized net loss due to lump sum settlements	122	0	2,412
Change in net actuarial loss	919	1,211	442
Recognized net loss due to defined benefit plan termination	0	10,907	0
Derivatives:
Change in the accumulated gain (loss) on effective cash flow hedge derivatives	373	122	(228)
Reclassification adjustment for (gain) loss realized in net income	(242)	174	370
Other comprehensive income (loss), net of tax	(7,430)	6,073	(404)
Comprehensive income	$ 142,154	$ 111,703	$ 108,007