CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Marketable securities, amortized cost	$ 6,415,588	$ 6,788,729
Estimated fair value	$ 806,942	$ 776,557
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)	42,644,544	42,467,394
Preferred Stock
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	188,500	6,104
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	$ 0	$ 0
Estimated fair value	$ 806,942	$ 776,557