Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Interest bearing deposits, banks	$ 53,654	$ 19,826
Held-to-maturity securities, fair values	$ 1,016,111	$ 1,023,784
Preferred stock, no par value
Preferred stock, shares authorized	1,000,000	1,000,000
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 2.0833	$ 2.0833
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	46,655,012	44,043,244
Common stock, shares outstanding	46,643,250	44,043,244
Treasury stock, shares	11,762,000	0