Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 31,248	$ 27,003
Unrealized gains on available for sale securities and transferred securities:
Unrealized holding gains arising during the period	1,411	21,825
Less: adjustment for net losses realized in net income	0	191
Change in net unrealized holding loss on securities transferred to held to maturity	761	1,682
Pension and other postretirement benefit plans:
Net change in prior service costs	39	38
Recognized net loss due to lump sum settlement	0	261
Change in net actuarial loss	486	545
Derivatives:
Change in the accumulated gain (loss) on effective cash flow hedge derivatives	35	(820)
Less: adjustment for loss realized in net income	61	99
Other comprehensive income, net of tax	2,793	23,821
Comprehensive income	$ 34,041	$ 50,824