Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Held to maturity securities, fair value	$ 8,656,409	$ 9,081,236
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	20,000,000	20,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	69,261,431	69,846,825
Common stock, shares outstanding (in shares)	69,261,431	69,846,825
Accumulated other comprehensive income (loss)—net unrealized gain (loss) on available for sale securities, tax expense (benefit)	$ 338	$ 82