CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 4,974,616	$ 4,946,278
Equity securities, cost	$ 4,143
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)	371,348,000	370,567,000
Common stock, shares outstanding (in shares)	371,348,000	370,567,000