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	$ 149,523	$ 108,032	$ 86,830
Other Comprehensive Income (Loss), Net of Tax:
Unrealized holding gains (losses) on securities available-for-sale arising during the period	1,269	(18,723)	(1,952)
Reclassification adjustment for gains and losses on realized income	7,388	(129)	(630)
Tax effect	(3,029)	6,598	904
Net unrealized gains (losses) on securities available-for-sale, net of tax	5,628	(12,254)	(1,678)
Unrealized gains on interest rate swaps designated as cash flow hedges	4,263	0	0
Reclassification adjustment for losses included in net income	1,633	0	0
Tax effect	(2,064)	0	0
Net unrealized gains on interest rate swaps designated as cash flow hedges, net of tax	3,832	0	0
Plan remeasurement	13,011	(707)	0
Adjustment for pension and other postretirement benefits	1,752	2,630	7,845
Tax effect	(5,167)	(673)	(2,746)
Net adjustment for pension and other postretirement benefits	9,596	1,250	5,099
Other comprehensive income (loss), net of tax	19,056	(11,004)	3,421
Total comprehensive income, net of tax	$ 168,579	$ 97,028	$ 90,251