Consolidated Statements of Condition (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Investment securities, available-for-sale, book value	$ 848,434	$ 807,761
Investment securities, held-to-maturity-at cost, fair value	162,853	163,232
Allowance for loan losses	$ 40,599	$ 39,751
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	65,000,000	65,000,000
Common stock, issued (in shares)	56,279,527	55,995,219
Treasury stock (in shares)	24,861,145	24,605,145