Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net Income	$ 171,753	$ 161,625	$ 149,502
Unrealized (losses) gains on available for sale investment securities:
Unrealized gain (loss) on securities	10,432	(14,891)	(7,717)
Reclassification adjustment for securities gains included in net income	(5,894)	(1,657)	(5,892)
Non-credit related unrealized gain (loss) on other-than-temporarily impaired debt securities	185	(185)	239
Net unrealized gains (losses) on available for sale investment securities	4,723	(16,733)	(13,370)
Unrealized gains on derivative financial instruments:
Amortization of unrealized loss on derivative financial instruments	0	16	75
Reclassification adjustment for loss on derivative financial instruments included in net income	0	0	2,456
Net unrealized gains on derivative financial instruments	0	16	2,531
Defined benefit pension plan and postretirement benefits:
Unrecognized pension and postretirement (cost) income	(609)	(931)	4,680
Amortization of net unrecognized pension and postretirement income	1,361	1,216	1,864
Net unrealized gains on defined benefit pension and postretirement plans	752	285	6,544
Other Comprehensive Income (Loss)	5,475	(16,432)	(4,295)
Total Comprehensive Income	$ 177,228	$ 145,193	$ 145,207