Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Fixed maturities, available for sale, amortized cost	$ 14,914,580	$ 14,188,050
Long-term debt, fair value	$ 1,243,232	$ 1,233,019
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	320,000,000	320,000,000
Common stock, shares issued (in shares)	127,218,183	127,218,183
Common stock, shares held in treasury (in shares)	11,859,162	9,187,075