CONSOLIDATED STATEMENTS OF CONDITION (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Assets:
Available-for-sale investment securities, cost	$ 2,761,022	$ 2,397,886
Shareholders' equity:
Preferred stock, par value (in dollars per share)	$ 1.00	$ 1.00
Preferred stock, authorized (in shares)	500,000	500,000
Preferred stock, issued (in shares)	0	0
Common stock, par value (in dollars per share)	$ 1.00	$ 1.00
Common stock, authorized (in shares)	75,000,000	75,000,000
Common stock, issued (in shares)	41,840,221	41,606,422
Treasury stock, shares at cost (in shares)	963,615	858,701