CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 65,384	$ 60,162	$ 44,530
Other comprehensive income (loss) net of tax:
Unrealized holding gain (loss) on securities available for sale arising during the period, net of tax of $20, $8,001, and $6,841	$ (37)	$ 14,860	(12,705)
Unrealized loss on securities transferred to held-to-maturity, net of tax of $0, $0, and $1,786			(3,316)
Unrealized gain on securities available for sale for which a portion of an other than temporary impairment has been recognized in income, net of tax of $0, $995, and $767		$ 1,847	1,425
Unrealized gain (loss) on cash flow hedges arising during the period, net of tax of $588, $1,397, and $831	$ (1,093)	(2,599)	1,543
Reclassification adjustment for net losses (gains) included in net income net of tax of $435, $760, and $157	(808)	(1,410)	291
Defined benefit pension plans, net of tax of $1,187, $4,264, and $6,382
Net gain (loss) arising during period	2,404	(7,580)	10,704
Amortization of prior service cost	(198)	(338)	1,147
Period change	268	4,780	(911)
Comprehensive income	$ 65,652	$ 64,942	$ 43,619