Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Thousands, $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Amortized cost on fixed maturities trading investments	$ 2,776,887	$ 2,795,410
Cost - equity security trading investments	479,410	331,770
Cost - short-term investments	366,512	375,700
Fixed assets, accumulated depreciation	$ 335,477	$ 319,429
Common Stock
Common stock, no par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	70,000	70,000
Common stock, shares issued (in shares)	55,332	55,289
Common stock, shares outstanding (in shares)	55,332	55,289