Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 95,636	$ 86,795	$ 84,401
Investment securities available-for-sale:
Change in net unrealized gains (losses) during the period	(19,379)	3,151	27,522
Reclassification adjustment for net gains included in income	(330)	(137)	(61)
Change in unamortized loss on available-for-sale investment securities transferred into held-to-maturity	1,858	1,611	(6,923)
Unrealized Gain (Loss) on Derivatives	(203)	165	0
Defined benefit post-retirement benefits plans:
Change in net actuarial loss	3,983	58	1,731
Other comprehensive income (loss), before tax	(14,071)	4,848	22,269
Deferred tax benefit (expense) related to other comprehensive income (loss)	5,537	(1,908)	(8,762)
Other comprehensive income (loss), net of tax	(8,534)	2,940	13,507
Total comprehensive income	$ 87,102	$ 89,735	$ 97,908