CONSOLIDATED STATEMENTS OF CONDITION (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Available-for-sale investment securities, cost	$ 3,005,165	$ 3,007,148
Shareholders' equity:
Preferred stock, par value (in dollars per share)	$ 1.00	$ 1.00
Preferred stock, shares authorized (in shares)	500,000	500,000
Preferred stock, shares issued (in shares)	0	0
Common stock, par value (in dollars per share)	$ 1.00	$ 1.00
Common stock, shares authorized (in shares)	75,000,000	75,000,000
Common stock, shares issued (in shares)	51,374,254	51,263,841
Treasury stock, shares at cost (in shares)	490,651	567,764
Shares held by deferred compensation arrangements (in shares)	203,730	237,494