Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 33,339	$ 36,830
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	14,118	(21,030)
Change in net unrealized holding loss on securities transferred to held to maturity	560	724
Pension and other postretirement benefit plans:
Net change in prior service costs	47	47
Recognized net loss due to lump sum settlement	24	31
Change in net actuarial loss	190	276
Derivatives:
Change in the accumulated gain (loss) on effective cash flow hedge derivatives	(47)	320
Reclassification adjustment for (gain) loss realized in net income	(128)	(5)
Other comprehensive income (loss), net of tax	14,764	(19,637)
Comprehensive income	$ 48,103	$ 17,193