CONSOLIDATED STATEMENTS OF CONDITION (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Available-for-sale investment securities, cost	$ 2,915,267	$ 2,952,278
Equity and other securities, cost	$ 42,241	$ 44,678
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)	51,727,758	51,576,839
Treasury stock, shares at cost (in shares)	256,387	319,015
Shares held by deferred compensation arrangements (in shares)	176,252	207,403