Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 42,140	$ 39,813	$ 75,479	$ 76,643
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	15,059	(6,955)	29,177	(27,985)
Change in net unrealized holding loss on securities transferred to held to maturity	942	728	1,502	1,452
Pension and other postretirement benefit plans:
Net change in prior service costs	47	46	94	93
Recognized net loss due to lump sum settlement	47	30	71	61
Change in net actuarial loss	186	272	376	548
Derivatives:
Change in the accumulated gain (loss) on effective cash flow hedge derivatives	(76)	99	(123)	419
Reclassification adjustment for (gain) loss realized in net income	(106)	(73)	(234)	(78)
Other comprehensive income (loss), net of tax	16,099	(5,853)	30,863	(25,490)
Comprehensive income	$ 58,239	$ 33,960	$ 106,342	$ 51,153