Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 27,003	$ 29,148
Unrealized gains on available for sale securities and transferred securities:
Unrealized holding gains arising during the period	21,825	11,386
Less: adjustment for net losses realized in net income	191	0
Change in net unrealized holding loss on securities transferred to held to maturity	1,682	874
Pension and other postretirement benefit plans:
Net change in prior service costs	38	39
Recognized net loss due to lump sum settlement	261	257
Change in net actuarial loss	545	754
Derivatives:
Change in the accumulated loss on effective cash flow hedge derivatives	(820)	(608)
Less: adjustment for loss realized in net income	99	130
Other comprehensive income, net of tax	23,821	12,832
Comprehensive income	$ 50,824	$ 41,980