Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 51,962	$ 22,218
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	(16,350)	30,380
Change in net unrealized holding loss on securities transferred to held to maturity	533	646
Reclassification adjustments for changes realized in net income:
Net change in prior service costs	21	28
Recognized net loss due to lump sum settlement	0	0
Change in net actuarial loss	341	244
Other comprehensive income (loss), net of tax	(15,455)	31,298
Comprehensive income	$ 36,507	$ 53,516