Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 24,035	$ 21,503	$ 55,283	$ 48,506
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	2,832	5,787	4,243	27,612
Less: adjustment for net (gains) losses realized in net income	(1)	0	(1)	191
Change in net unrealized holding loss on securities transferred to held to maturity	734	1,836	1,495	3,518
Pension and other postretirement benefit plans:
Net change in prior service costs	38	39	77	77
Recognized net loss due to lump sum settlement	0	1,388	0	1,649
Change in net actuarial loss	482	540	968	1,085
Recognized net loss due to defined benefit plan termination	10,492	0	10,492	0
Derivatives:
Change in the accumulated gain (loss) on effective cash flow hedge derivatives	(129)	(277)	(94)	(1,097)
Less: adjustment for loss realized in net income	48	96	109	195
Other comprehensive income (loss), net of tax	14,496	9,409	17,289	33,230
Comprehensive income	$ 38,531	$ 30,912	$ 72,572	$ 81,736