CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Fixed income, amortized cost	$ 6,317,164	$ 5,562,550
Equity securities, cost	$ 17,522	$ 17,188
Shareholders' equity:
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	1,000,000,000	1,000,000,000
Common stock, shares issued (in shares)	371,353,000	371,353,000
Common stock, shares outstanding (in shares)	338,573,000	347,308,000
Treasury stock, shares at cost (in shares)	32,779,000	24,045,000