CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Fixed income, amortized cost	$ 5,196,784	$ 4,946,278
Equity securities, cost	$ 3,993
Equity securities, cost		$ 7,223
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	370,567,000
Common stock, shares outstanding (in shares)	355,371,000	370,567,000
Treasury stock, shares at cost (in shares)	15,982,000	0