Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 22,218	$ 33,339
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	30,380	14,118
Change in net unrealized holding loss on securities transferred to held to maturity	646	560
Reclassification adjustments for changes realized in net income:
Net change in prior service costs	28	47
Recognized net loss due to lump sum settlement	0	24
Change in net actuarial loss	244	190
Derivatives:
Change in the accumulated gain (loss) on effective cash flow hedge derivatives	0	(47)
Reclassification adjustment for (gain) loss realized in net income	0	(128)
Other comprehensive income (loss), net of tax	31,298	14,764
Comprehensive income	$ 53,516	$ 48,103