Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Consolidated Statements of Comprehensive Income (unaudited) [Abstract]
Net income	$ 25,986	$ 20,279
Other comprehensive (loss) income, net of tax:
Unrealized net holding (losses) gains arising during the period (pre-tax amounts of ($15,454) and $836)	(11,590)	497
Reclassification adjustment for an impairment write-down of equity security (pre-tax amounts of $- and $1,312)	0	811
Unrealized gains on derivatives (cash flow hedges) (pre-tax amounts of $1,048 and $249)	786	154
Reclassification of net unrealized (gains) losses on cash flow hedges to interest expense (pre-tax amounts of $(359) and $82)	(269)	50
Amortization of unrealized net gains related to the reclassification of available for sale investment securities to held to maturity (pre-tax amounts of $188 and $238)	141	147
Pension and other benefits:
Amortization of prior service cost and actuarial gains (pre-tax amounts of $295 and $435)	221	269
Total other comprehensive (loss) income	(10,711)	1,928
Comprehensive income	$ 15,275	$ 22,207