Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 30,982	$ 28,430	$ 79,488	$ 88,180
Unrealized (losses) gains on available for sale securities and transferred securities:
Unrealized holding (losses) gains arising during the period	(7,816)	11,035	19,796	8,470
Less: adjustment for net losses realized in net income	0	0	191	0
Change in net unrealized holding loss on securities transferred to held to maturity	1,653	1,036	5,171	2,931
Pension and other postretirement benefit plans:
Net change in prior service costs	39	39	116	116
Recognized net loss due to lump sum settlement	286	373	1,935	926
Change in net actuarial loss	573	751	1,658	2,256
Derivatives:
Change in the accumulated loss on effective cash flow hedge derivatives	257	(751)	(840)	(1,185)
Less: adjustment for loss realized in net income	97	130	292	390
Other comprehensive (loss) income, net of tax	(4,911)	12,613	28,319	13,904
Comprehensive income	$ 26,071	$ 41,043	$ 107,807	$ 102,084