Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Consolidated Balance Sheets [Abstract]
Securities available for sale, amortized cost	$ 2,949,057	$ 2,562,000
Securities held to maturity, fair value	2,962,010	2,470,117
Property and equipment, accumulated depreciation	$ 214,998	$ 231,127
Common stock, par value per share	$ 3.33	$ 3.33