Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Securities available for sale, amortized cost	$ 2,758,723	$ 2,755,806
Securities held to maturity, fair value	2,979,401	2,935,856
Property and equipment, accumulated depreciation	238,051	$ 225,969
Right of use assets, accumulated amortization	$ 6,172
Preferred stock, par value per share	$ 20.00	$ 20.00
Common stock, par value per share	$ 3.33	$ 3.33