CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Securities carried at fair value, cost	$ 6,357,582	$ 6,024,225
Held to maturity, allowance for credit losses	48	51
Estimated Fair Value	$ 95,082	$ 100,041
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	60,000,000	60,000,000
Common stock, shares issued (in shares)	42,173,675	42,093,862
Preferred Stock
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	6,104	6,104
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Series A Junior Participating preferred stock
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	16,000	16,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Series B Fixed Rate Cumulative Perpetual Preferred Stock
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	81,698	81,698
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Series C Senior Non-Cumulative Perpetual Preferred Stock
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	81,698	81,698
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Series D Senior Non-Cumulative Perpetual Convertible Preferred Stock
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	3,000	3,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Series E Fixed-Rate Reset Cumulative Perpetual Preferred Stock
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	11,500	11,500
Preferred stock, shares issued (in shares)	11,500	11,500
Preferred stock, shares outstanding (in shares)	11,500	11,500
Obligations of states and political subdivisions
Held to maturity, allowance for credit losses	$ 48	$ 51
Estimated Fair Value	$ 95,082	$ 100,041