Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Amortized cost on fixed maturities trading investments	$ 2,823,230	$ 2,795,410
Cost - equity security trading investments	474,197	331,770
Cost - short-term investments	$ 302,693	$ 375,700
Common Stock
Common stock, no par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	70,000,000	70,000,000
Common stock, shares issued (in shares)	55,332,000	55,289,000
Common stock, shares outstanding (in shares)	55,332,000	55,289,000