Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Fixed maturities, available for sale, amortized cost	$ 14,995,101	$ 14,188,050
Long-term debt, fair value	$ 1,228,392	$ 1,233,019
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized	320,000,000	320,000,000
Common stock, shares issued	124,218,183	127,218,183
Common stock, shares held in treasury	9,625,104	9,187,075