Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Consolidated Statements of Comprehensive Income (unaudited) [Abstract]
Net income	$ 20,279	$ 18,891
Other comprehensive income, net of tax
Unrealized net holding gains arising during the period (pre-tax amounts of $836 and $13,211)	497	8,072
Reclassification adjustment for net gains (losses) related to securities available for sale included in net income (pre-tax amounts of $- and $29)	0	(19)
Reclassification adjustment for an impairment write-down of equity security (pre-tax amounts of $1,312 and $-)	811	0
Unrealized gains on derivatives (cash flow hedges) (pre-tax amounts of $331 and $-)	204	0
Amortization of unrealized net gains related to the reclassification of available for sale investment securities to held to maturity (pre-tax amounts of $238 and $296)	147	181
Pension and other benefits:
Amortization of prior service cost and actuarial loss (pre-tax amounts of $435 and $512)	269	313
Total other comprehensive income	1,928	8,547
Comprehensive income	$ 22,207	$ 27,438