Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Fixed maturities, available-for-sale, amortized cost	$ 22,696,473	$ 22,762,962
Fixed maturities, held for trading, amortized cost	142,981	20,512
Mortgage loans on real estate, allowances	773	773
Short-term investments, available-for-sale, amortized cost	$ 415,189	$ 350,266
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, number of shares authorized	50,000,000	50,000,000
Preferred stock, number of shares issued	0	0
Preferred stock, number of shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, number of shares authorized	50,000,000	50,000,000
Common stock, number of shares issued	7,320,176	7,320,176
Common stock, number of shares outstanding	7,320,176	7,320,176