Statements of Financial Position (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Fixed maturities, available-for-sale, amortized cost	$ 10,145,266	$ 9,818,298
Equity securities, available-for-sale, cost	$ 14	$ 365
Common stock, par value (in dollars per share)	$ 100	$ 100
Common stock, shares authorized	25,000	25,000
Common stock, shares issued	25,000	25,000
Common stock, shares outstanding	25,000	25,000