CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 5,042,011	$ 4,946,278
Equity securities, cost	$ 4,134
Equity securities, cost		$ 7,223
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)	362,155,000	370,567,000
Treasury stock, shares at cost (in shares)	9,198,000	0