Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 32,150	$ 42,140	$ 54,368	$ 75,479
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	1,986	15,059	32,366	29,177
Change in net unrealized holding loss on securities transferred to held to maturity	583	942	1,229	1,502
Reclassification adjustments for changes realized in net income:
Net change in prior service costs	28	47	56	94
Recognized net loss due to lump sum settlement	30	47	30	71
Change in net actuarial loss	240	186	484	376
Derivatives:
Change in the accumulated gain (loss) on effective cash flow hedge derivatives	0	(76)	0	(123)
Reclassification adjustment for (gain) loss realized in net income	0	(106)	0	(234)
Other comprehensive income (loss), net of tax	2,867	16,099	34,165	30,863
Comprehensive income	$ 35,017	$ 58,239	$ 88,533	$ 106,342