CONSOLIDATED STATEMENTS OF CONDITION (Unaudited) (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Assets:
Available-for-sale investment securities, cost	$ 1,708,296	$ 1,989,938
Held-to-maturity investment securities, fair value	$ 660,415	$ 703,957
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)	40,881,188	40,421,493
Treasury stock, shares at cost (in shares)	782,173	795,560