CONSOLIDATED STATEMENTS OF CONDITION (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets:
Available-for-sale investment securities, cost	$ 3,898,704	$ 3,427,779
Equity and other securities, cost	$ 44,138	$ 46,511
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)	54,019,374	53,754,599
Treasury stock, shares at cost (in shares)	144,395	161,472
Shares held by deferred compensation arrangements (in shares)	144,380	161,457