Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 86,795	$ 84,401	$ 86,136
Investment securities available-for-sale:
Change in net unrealized gains (losses) during the period	5,863	21,147	(52,949)
Reclassification adjustment for net gains included in income	(137)	(61)	(1)
Change in unamortized loss on available-for-sale investment securities transferred into held-to-maturity	(1,101)	(548)	0
Unrealized Gain (Loss) on Derivatives	165	0	0
Defined benefit post-retirement benefits plans:
Change in net actuarial loss	58	1,731	137
Other comprehensive income (loss), before tax	4,848	22,269	(52,813)
Deferred tax benefit (expense) related to other comprehensive income (loss)	(1,908)	(8,762)	20,781
Other comprehensive income (loss), net of tax	2,940	13,507	(32,032)
Total comprehensive income	$ 89,735	$ 97,908	$ 54,104