Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 82,151	$ 78,409	$ 76,425
Other comprehensive (loss) income, net of tax:
Unrealized net holding (losses) arising during the year (pre-tax amounts of $(6,915), $(8,618) and $(3,159))	(4,070)	(5,265)	(1,930)
Reclassification adjustment for net (gains) losses related to securities available for sale included in net income (pre-tax amounts of ($1,869), $644 and ($3,087))	(1,153)	393	(1,886)
Reclassification adjustment for an impairment write-down of an equity security (pre-tax amounts of $1,312, $- and $-)	811	0	0
Unrealized gains on derivatives (cash flow hedges) (pre-tax amounts of $609, $2,901 and $-)	372	1,772	0
Amortization of unrealized net gains and losses related to the reclassification of available for sale securities to held to maturity (pre-tax amounts of $875, $1,094 and $1,311)	540	668	801
Pension and other benefits:
Amortization of prior service cost and actuarial gains (pre-tax amounts of $1,852, $2,370 and $2,239)	1,112	1,421	1,371
Decrease (increase) in unrecognized actuarial loss (pre-tax amounts of $2,401, $3,154 and $(6,144))	1,831	1,909	(3,747)
Total other comprehensive (loss) income	(557)	898	(5,391)
Comprehensive income	$ 81,594	$ 79,307	$ 71,034