Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 41,035	$ 36,253	$ 116,514	$ 112,896
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	3,116	(7,246)	32,293	(35,231)
Change in net unrealized holding loss on securities transferred to held to maturity	637	730	2,139	2,182
Pension and other postretirement benefit plans:
Net change in prior service costs	46	47	140	140
Recognized net loss due to lump sum settlement	34	31	105	92
Change in net actuarial loss	186	271	562	819
Derivatives:
Change in the accumulated gain (loss) on effective cash flow hedge derivatives	14	39	(109)	458
Reclassification adjustment for (gain) loss realized in net income	(75)	(79)	(309)	(157)
Other comprehensive income (loss), net of tax	3,958	(6,207)	34,821	(31,697)
Comprehensive income	$ 44,993	$ 30,046	$ 151,335	$ 81,199