CONSOLIDATED STATEMENTS OF CONDITION (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Available-for-sale investment securities, cost	$ 3,068,406	$ 3,011,551
Equity and other securities, cost	$ 46,419	$ 42,965
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)	53,697,919	51,974,726
Treasury stock, shares at cost (in shares)	160,326	180,803
Shares held by deferred compensation arrangements (in shares)	160,311	179,548