CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Fixed income, amortized cost	$ 5,562,550	$ 5,196,784
Equity securities, cost	$ 17,188	$ 3,993
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)	347,308,000	355,371,000
Treasury stock, shares at cost (in shares)	24,045,000	15,982,000