Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Securities available for sale, amortized cost	$ 2,698,865	$ 2,755,806
Securities held to maturity, fair value	2,892,910	2,935,856
Property and equipment, accumulated depreciation	233,078	$ 225,969
Right of use assets, accumulated amortization	$ 3,064
Preferred stock, par value per share	$ 20.00	$ 20.00
Common stock, par value per share	$ 3.33	$ 3.33