Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 47,981	$ 32,150	$ 99,943	$ 54,368
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	4,959	1,986	(11,391)	32,366
Change in net unrealized holding loss on securities transferred to held to maturity	552	583	1,085	1,229
Reclassification adjustments for changes realized in net income:
Net change in prior service costs	21	28	42	56
Recognized net loss due to lump sum settlement	0	30	0	30
Change in net actuarial loss	333	240	674	484
Other comprehensive income (loss), net of tax	5,865	2,867	(9,590)	34,165
Comprehensive income	$ 53,846	$ 35,017	$ 90,353	$ 88,533