Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 105,630	$ 108,411	$ 116,038
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	(8,641)	(9,667)	(10,309)
Less: adjustment for net (gains) losses realized in net income	(9)	191	0
Change in net unrealized holding loss on securities transferred to held to maturity	2,915	6,070	3,918
Pension and other postretirement benefit plans:
Net change in prior service costs	154	154	154
Recognized net loss due to lump sum settlements	0	2,412	1,371
Change in net actuarial loss	451	294	2,252
Recognized net loss due to defined benefit plan termination	10,907	0	0
Derivatives:
Change in the accumulated gain (loss) on effective cash flow hedge derivatives	122	(228)	(812)
Less: adjustment for loss realized in net income	174	370	516
Other comprehensive income (loss), net of tax	6,073	(404)	(2,910)
Comprehensive income	$ 111,703	$ 108,007	$ 113,128