Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Consolidated Statements of Comprehensive Income (unaudited) [Abstract]
Net income	$ 22,876	$ 20,001	$ 64,514	$ 58,801
Other comprehensive income, net of tax:
Unrealized net holding gains (losses) arising during the period (pre-tax amounts of $1,355, $(2,931), $4,719 and $14,010)	837	(1,790)	2,896	8,560
Reclassification adjustment for net losses (gains) related to securities available for sale included in net income (pre-tax amounts of $(4), $-, $(2) and $30)	2	0	1	(19)
Reclassification adjustment for an impairment write-down of equity security (pre-tax amounts of $-, $-, $1,312 and $-)	0	0	811	0
Unrealized (loss) gain on derivatives (cash flow hedges) (pre-tax amounts of $(63), $782, $(167) and $719)	(40)	478	(103)	439
Amortization of unrealized net gains related to the reclassification of available for sale investment securities to held to maturity (pre-tax amounts of $212, $267, $675 and $843)	131	162	417	515
Pension and other benefits:
Amortization of prior service cost and actuarial loss (pre-tax amounts of $480, $519, $1,350, $1,544)	297	317	834	943
Total other comprehensive income (loss)	1,227	(833)	4,856	10,438
Comprehensive income	$ 24,103	$ 19,168	$ 69,370	$ 69,239