Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Consolidated Statements of Comprehensive Income (unaudited) [Abstract]
Net income	$ 18,891	$ 18,166
Other comprehensive income, net of tax
Unrealized net holding gains arising during the period (pre-tax amounts of $13,211 and $4,842)	8,072	2,959
Reclassification adjustment for net gains related to securities available for sale included in net income (pre-tax amounts of $29 and $14)	(19)	(9)
Amortization of unrealized net gains and losses related to the reclassification of available for sale investment securities to held to maturity (pre-tax amounts of $296 and $307)	181	204
Pension and other benefits:
Amortization of prior service cost and actuarial gains (pre-tax amounts of $512 and $561)	313	342
Total other comprehensive income	8,547	3,496
Comprehensive income	$ 27,438	$ 21,662